Goat farming is a thriving agricultural business in India, which opens huge employment opportunities and livelihood for rural communities. Breeding and reproduction play pivotal role in goat enterprise, in which early pregnancy diagnosis and assessment of foetal development is crucial in deciding the management practice. The diagnosis of non-pregnant does early in the breeding season is the first stage in the reproductive herd health program. Pregnant and non-pregnant animals can be distinguished to reduce reproductive and productive losses.  Ultrasonography is a commonly practiced and widely accepted non- invasive technique to detect pregnancy and foetal viability (Karen et al., 2009 and Amer, 2010). During gestation, Doppler ultrasound offers a pattern of blood flow in the organ of interest, such as velocity, circulation and blood type, which has critical clinico-gynecological significance in animal species. Compared to companion animals and other species, the Doppler technique has not been widely used in goat obstetrics and only a few studies have investigated Doppler predictors in aberrant gestation.
       
Doppler ultrasonography measures velocity indices and blood flow volume to assess vascular perfusion. The haemodynamic changes of umbilical cord vascular system provide blood flow to the developing placenta and foetus and poor uterine perfusion has been identified as the leading cause of prenatal growth restriction. However, despite progress in ultrasonographic methodologies, there exists a notable deficiency in the thorough documentation and standardization of hemodynamic indices and blood flow parameters, especially concerning the uterine and umbilical arteries in goats across various breeds and gestational periods. Research examining the relationship between these indices and the early identification of gestational disorders is scarce, creating a significant gap in understanding how these parameters can be utilized for clinical diagnostics and treatment in caprine species. This review emphasizes the crucial contribution of ultrasonography and Doppler techniques in enhancing our comprehension of fetal development and maternal hemodynamics in goats. By synthesizing current knowledge and pinpointing research deficiencies, the article highlights the necessity for ongoing exploration in this field, which could greatly improve reproductive management strategies, enhance neonatal health outcomes and enable the early detection of gestational abnormalities in goats.

Maternal and foetal echobiometry of caprine pregnancy
 
B-mode real-time ultrasonography is a reliable, quick and easy imaging approach for detecting pregnancy in goats with approximately 100 per cent accuracy (Buckrell, 1988 and Dhillon et al., 2020). To achieve high-quality images, it is essential to select the appropriate probe type (linear, sector, or convex) and frequency (3.5, 5, or 7.5 MHz), particularly as transrectal palpation is infrequently employed in small ruminants, making this choice even more critical. B-mode ultrasonography is generally used for biometric and foetal morphological examinations.
       
An effective method for determining the stage of pregnancy is ultrasound foetal biometry. In Saanen goats (Abdelghafar et al., 2011), Japanese miniature Shiba goats (Kandiel et al., 2015) and Jamnapari goats (Abubakar et al., 2016), the crown to rump length, biparietal, femur and trunk were correlated with the gestational age. Ultrasonography examinations can be performed in all the stages of pregnancy. Small ruminant gestation can be divided into three stages for the foetometric measurements. The first stage lasts for 50 days, the second stage lasts for 50-100 days and the third stage accounts for more than 100 days till parturition (Erdogan, 2012).
 
Uterine lumen diameter
 
The uterine lumen diameter is measured during early pregnancy in goats. It can be measured from the 21^st day of gestation until the end of the first stage (Karadaev et al., 2016).  According to Padilla-Rivas et al. (2005), a tiny, fluid-filled, non-echogenic vesicle measuring about 1 cm in diameter was observed in the uterine lumen of Boer goats by day 21. Amer (2010) verified that the transverse diameter of the embryonic vesicle was around 10 mm when it was initially observed in dairy goats at gestational day 19.5± 0.30. According to Karadaev et al., (2018), the first metric measured (10.15±2.39 mm) in Bulgarian goats on the 21^st day of gestation was the increased uterine lumen filled with anechoic amniotic fluid.
 
Crown rump length (CRL)
 
Crown rump length is defined as the length of the entire foetus, measured from the top of the foetal skull to the end of the sacrum. When the foetus is in a curved position, CRL is measured first from the head to the heart and then from the heart to the end of the sacrum (Abdelghafar et al., 2011 and Karadaev et al., 2018). Madhusudhan (2017) reported that the average foetal CRL measurements in Malabari does were 2.33±0.009 to 15.18±0.064 cm on days 30, 44, 59, 74 and 89 of gestation. The equation formulated for gestational age calculation was y=24.226+4.529x, where x is the foetal CRL (R² = 0.968). CRL in goats can be measured from the 21^st to 49^th day of gestation (Karadaev et al., 2018). The average CRL of the foetus on days 30, 45 and 60 of gestation was 1.38±0.11, 3.74±0.26 and 7.7± 0.24 cm in singleton pregnancies and 1.32±0.08, 3.90± 0.20 and 7.6±0.18 cm in multiple foetal pregnancies, according to Vinayak (2015). He concluded that the CRL not significant difference between singleton and multiple pregnancies.
       
According to Saeedipanah et al., (2022), the straight CRL in Baluchi sheep was demarcated as the longest straight length of the foetal mass between the occipital bone and the rump. The CRL was measured transrectally from days 19 to 55 and transabdominally from days 81 to 83.  The formula for calculating gestational age in Baluchi sheep is Y^days = 21.537 + 0.3722 CRL^mm.
 
Bi-parietal diameter (BPD)
 
In goats, the BPD of the foetal skull is measured from the first stage of pregnancy itself. The cerebral falx midline should divide the hemispheres into two equidistant sections when taking the BPD and the measurement is taken from the outer surface of the proximal calvarium to the inner surface of the distal calvarium (Amer, 2008). Kandiel et al., (2015) found that the braincase diameter of Miniature Shiba goat foetuses increased dramatically from 10.50±0.17 mm to 64.00±0.91 mm at the first to fourth month and that it has strong positive correlation (R2=0.98) with gestational age. Biparietal diameter as shown to be the most reliable variable for identifying gestational age in sheep (R²=95.0) compared to goats (R²=96.5) (Ejaz-ul-Haq et al., 2020), where it was measurable from 34 to 125 days with R² = 83.0. Gouda et al. (2021) reported that the mean biparietal diameter could be accurately measured between 6^th to 16^th gestational weeks 20.00±0.05 and 64.00±0.00 to estimate the gestational age in Baladi goats. Due to its extended application time and strong correlation (R2=0.96) with gestational duration, BPD is a better option for small ruminants than CRL.
 
Occipito nasal length (ONL)/Occipito-snout diameter (OSD)
 
According to Yazici et al., (2018), Occipito Nasal Length (ONL) in Turkish Sannen goats can be measured as early as day 37 of pregnancy. The ONL and gestational age showed strong correlations (r=0.99), consistent with previous studies on sheep (Gonzalez de Bulnes et al., 1998; Gurler and Kaymaz, 2011) and goats (Kandiel et al., 2015). The ONL’s linear regression equation was y = -30+1.07x, with little difference between singleton and twins pregnancies in Turkish Saanen goats. Santos et al., (2018), who could measure ONL from sixth week of pregnancy in Santa Ines sheep.  A positive correlation was obtained between gestational age and ONL (r = 0.9873) and regression equations generated as y = 9.471x + 25.66 respectively. It concluded that ONL was precise over BPD in predicting parturition dates in Nellore Brown ewes. According to Saeedipanah et al., (2022), the OSD was indicated as the best parameter for predicting gestational age between days 36 and 109 in Baluchi sheep due to the lowest residuals, highest correlation coefficient. The regression equation y = 24.79 + (0.8271 ×  OSD ^mm) had a high correlation coefficient (R2 = 0.981) in Baluchi sheep.
 
Thoracic diameter
 
Suguna et al., (2008) used B- mode ultrasonography to evaluate the thoracic diameter in crossbred does during pregnancy and concluded that the thoracic diameter significantly increased as the gestational stage progressed. There was a high correlation between gestational age and thoracic diameter (r = 0.99). The formula derived for thoracic diameter regression was y = 26.0 (x ^cm) -4.8. Silva et al. (2021) used a linear regression model to determine the gestational age of Saanen goats having a coefficient (R2) of more than 90 per cent using parameters. Abdominal diameter and Thoracic diameter had strong predictive power for gestational age (R²=93.4 and R²=92.0). So, both parameters can used as a reliable estimators of the Saanen goat’s gestational stage. The conjugated equation for determining gestational age in Saanen goats was GA = 22.2152 + 0.892503 x BPD^mm + 0.936015 x TCD^mm (R2%=95.05).
       
In Baluchi sheep, the thoracic diameter was measured at two levels by Saeedipanah et al., (2022), one at the central (TDC) and other at final rib junction (TDR) on days 36-109 and 36-138, respectively. Foetal age and TDC (R² = 0.958) and TDR (R² = 0.975) showed a positive significant connection in Baluchi sheep. The regression equations obtained were Y^days = 24.933 + (2.1279 × TDC^mm) and Y^days = 31.961 + (1.2103 + TDR^mm), respectively. According to Muhammad and Aziz (2022), the foetus’s chest diameter in Shami goats increased drastically from 11.52 ± 0.84 mm on days 51–60 of pregnancy to 56.35 ± 0.93 mm on days 101–110. They observed a high positive correlation between gestational age and chest diameter in this breed (r=0.935) and the regression line of equation was Y^days=1.4470 X^mm+37.565.
 
Foetal trunk diameter
 
The foetal trunk diameter (FTD), one of the greatest diameters of the foetal body was measured from the ventral aspect of the spinal column to the ventral aspect of the abdomen. The formula derived for FTD in Korean goats was Y=13.2648 X (cm) + 33.7059 (Lee et al., 2005). In one study, Kandiel et al., (2015) measured the FTD during pregnancy and found that it was increased significantly as the pregnancy progressed, from 16.32±1.61 mm to 61.25 ±3.27 mm at first to fourth month, with a correlation coefficient of R² = 0.8969. While Karadaev et al., (2016) reported that the TD in Bulgarian local goats was 6.8±0.9 mm on day 28 of gestation and 18.8±1.1 mm at the end of the first trimester of pregnancy using transrectal ultrasonography with a linear transducer (5.0-12.0 MHz).
Madhusudhan (2017) assessed the mean FTD (cm) in Malabari goats between days 44 and 114 days of gestation and reported a consistent increase as 1.48±0.007 to 5.99 ±0.014, respectively. The regression formula derived; GA= 24.63+14.93 TD could be used to predict the gestational age of Malabari goat foetuses with high (R²=0.998) correlation coefficient.

Eye orbit diameter
 
The eye orbit diameter (EOD) is used to estimate the gestational age of goat foetuses between 57 and 124 days of gestation. Once the eye orbit is identified in ultrasonography in a rounded shape at its greatest size, the diameter of the circle created is measured (Erdogan, 2012).  Nwaogu et al., (2010) found the orbital, biparietal and occipitonasal diameters all exhibited a positive correlation (R = 0.97, 0.98 and 0.92) and high significance (P<0.001) with gestational age. The growth equations that were derived were GA = 22.881 + 6.668 ONL, GA = 26.938 + 14.30 BPD and GA = 17.326 +43.534 OD.
       
According to Kandiel et al., (2015), the foetal orbit diameter increased from 12.24 ± 0.93 mm in the first month of gestation to 69.62 ± 6.29 mm in the fourth month, indicating a strong correlation (R2=0.9239) between the two variables in Shiba goats. The linear regression equation Y^days = 1.6628 x ^cm - 5.2055. By using ultrasound examination on the 49^th day of pregnancy, Karadaev et al., (2016) discovered that the average foetal orbit diameter in Bulgarian local goats was 6.5 ± 0.5 mm. In Murciano- Granadina goat breed gestation was assessed between 75-125 days of post-breeding using the regression model, Y^days = 60.9832 x ^cm +10.4719 and the mean measurement of eye orbit diameter was 1.21±0.03 - 2.00±0.03 cm between 75-125 days of gestation (Ramírez-González et al., 2023).
 
Placentomes diameter
 
As the pregnancy continues, the concave round shape of the small ruminant placentomes produces C-shaped or O-shaped gray pictures against the black uterine fluid, depending on the plane of section. The placentome diameters (PD) were measured from the 18^th to 20^th day of pregnancy in Baluchi sheep and at least five large placentomes were required for the precise assessment. However, it had the lowest correlation coefficient with gestational age among the parameters tested as several placentomes appeared in the same picture in ultrasonography and the size of each placentome varied with gestational age (Saeedipanah et al., 2022).
       
Meanwhile Suguna (2008) reported a strong correlation (r = 0.99) between placentome diameter and gestational age in crossbred does, having placentome diameter increased dramatically from day 42 to day 130 of gestation (0.97±0.1 cm to 3.07±0.2 cm). Yaseen Mahmood (2016), observed a significant difference in PD between single and twin pregnancies of Iraq local goat breeds. The average PD size in single and twin-pregnant does were 7.5±0.41mm and 9.2±0.74 mm, respectively between days 35 to 45 of gestation. The maximal size of placentome was 39.2±2.50 mm in singleton-bearing does during 116-125 days and 41.0±2.19 mm in twin bearing does during 126-135 days of gestation. However, determination of gestation age based on placentome measurement beyond day 90 was not trustworthy.
       
Kuru et al., (2018) also observed a significant correlation (P<0.001) between placentome diameter and gestational age in Abaza and Gurcu goats. This study were similar findings to those of Karen et al., (2009) (R2 = 0.905) and Kandiel et al., (2015) (R2 = 0.899) and higher than those of Lee et al., (2005) (r = 0.574) and Nwaogu et al., (2010) (r = 0.45).
       
Vinayak (2015) compared the PD between single and multiple pregnancies during 45 to 135 of gestation in Malabari goats. The mean value of PD in singleton ranged from 1.14 ± 0.02 to 3.28±0.02 and in multiple pregnancies were 1.13 ± 0.01 to 3.25 ± 0.01 cm, respectively. The regression equation for estimation of gestational age in Malabari goats was Y= -4.757+41.564 × ^cm (R2 = 0.959). He concluded that, there was no significant between single and multiple pregnancies but significant difference was observed between the days (p≤0.05).
       
According to Saeedipanah et al., (2022), there was a substantial, nonlinear correlation between placental development and gestational age in Baluchi sheep. There was a strong correlation (p<0.001) between foetal age and BPD (between 36 -96 days) (R² = 0.961), CRL (R² = 0.935), FL (R² = 0.950), OSD (R² = 0.981) and TD (R² = 0.975). BPD and Placental diameter were used to measure significant non-linear correlation with age.
 
Measurements of foetal heart rate
 
Foetal heart rate was measured following the methods of Curran and Ginther (1995) and Karen et al., (2009). Serin et al., (2010) state that although the Foetal Heart Rate (FHR) was decreased during pregnancy, this was not evident until the final two weeks of the pregnancy. Specifically, between 130 and 145 days of pregnancy, the average FHR value dropped to 140 ± 6.4 bpm (P<0.05). The FHR values for multiple pregnancies and singleton pregnancies did not change (P>0.05). Erdogan et al. (2012) used B-mode doppler ultrasonography to measure the foetal heart rate (FHR) in Karya ewes. For multiple pregnancies, the average FHR during late gestation was 140.5±3.4 bpm, while for singleton conceptions, it was 141.70±5.4 bpm. In healthy pregnancies, the FHR values were above 220 bpm in the first trimester and below 190 bpm in the third. Jyothi et al., (2021), all foetuses showed cardiac movements in the chest area for the first time on days 36 and 43 in Nellore ewes. However, heartbeats were found earlier than this study (19-24 days) by Padilla-Rivas et al. (2005), Karen et al., (2009) and Kumar et al., (2015).
 
Measurement of heart axis length
 
According to Lee et al., (2005), the imaging of the foetal heart atria and ventricles in transverse axial projection during diastole and no movement of the foetus, the longitudinal (long) and transverse (short) heart axes were measured. The longitudinal and transverse heart axis had the greatest significant correlation to gestational age in Korean black goats. A study by Airina et al. (2011) examined the measurements of the foetal heart size in goats of mixed breeds of does at 7^th to 21^st week of gestation during both singleton and multiple pregnancies. The heart size for the single and twin pregnancy was 0.36±0.07 to 7.90±1.22 (cm²) and 0.41± 0.01 to 14.09± 1.24 in cross bred does. The coefficient of determination of R² in polynomial regression in Jermasia (R2 = 0.931) was found to be slightly higher than those in linear regression (R² = 0.910), which may be due to the polynomial growth pattern of the foetus’s foetal heart during pregnancy.
       
Kandiel et al., (2015), the longitudinal (long) and transverse (short) axis of the foetal heart in Shiba goats during pregnancy days increased from first months were 6.43±0.17 mm and 4.83±0.41 mm to 35.71±1.13 mm and 25.68 ± 0.70 mm in fourth months, respectively. A linear regression line equated with y = 3.1536x - 14.748 and y = 2.0632x - 8.2609, which were highly correlated with gestation R² = 0.9382 and 0.9589, respectively. According to Karadaev et al., (2021), the foetal heart short axis had values greater than those of the external one (R² = 0.95, 0.97, 0.97), while the long axis of foetal heart had the following values for R² = 0.94, 0.96, 0.96 and for SEE = 7.0, 5.5, 5.5 days for the linear, quadratic and exponential correlations, respectively.
 
Measurements of humerus and femur length
 
Usually, humerus is identified following the location of foetal heart and scapula and the femur is confirmed by identifying the foetus urinary bladder. For measuring the foetal development, Kahn (2004) measured the humerus and femur length in Saanen goats on weekly basis starting from week 8 till the parturition and the lengths of the bones were measured according to the zones of intensive calcification which appeared echo-rich. In Saanen goats, measurements of the humerus and femur were reported to be useful for predicting gestational age in the second and third trimesters (Abdelgafar et al., 2012).
       
According to Kahn (2004) the femur length and gestational age had higher correlations and coefficients of determination and femur length was a good predictor of foetal age in Kari sheep. The 6^th and 18^th weeks had the smallest and largest femur lengths of 17 and 45 mm, respectively. Also, in Yankasa ewes, femur length and gestational age had a significant connection (R2=0.9871) from days 60 to 103 of pregnancy which can be reliably during the second and third trimesters (James et al., (2015). 
       
Yadav (2021) measured the length of the foetus’ femur to estimate the gestational age of 21 pregnant Bangladeshi goats. Femur length (FL) was 37.2 mm and their real gestational age was 80 days. There was a significant difference between the estimated and actual gestational ages regarding FL (P≤0.05). The following formulas were created to predict the gestational age of Bangladeshi goats: GA=1.27+0.98CRL,GA=22.99+1.94BPD, GA=34.67+1.35FL and GA= 4.43PD-37.41, with coefficients of determination (R² = 98, 96, 91 and 63) accordingly. Based on the current study, it can be said that the CRL and BPD were very reliable for estimating GA in the first and second trimesters, whereas FL is advised in the third.
 
Doppler ultrasonography of umbilical artery, uterine artery and foetal aorta
 
The study on the pulsatility of umbilical artery by Surat and Adamson (1996) implied a correlation between the vascular walls and vascular resistance of placenta. As the fetal size increases, the arteries develop greater vascular musculature that decreases their radius and results in a progressive decrease in pulsatility index (PI). In other studies (Serin et al., 2010 and Martínez-Díaz et al., 2022) the resistance index (RI) values decreased progressively and significantly throughout the pregnancy corresponding to the increased requirements of foetus due to its rapid growth. As a result, low resistance of the umbilical arteries favored increased blood flow to the umbilical cord. Beltrame (2017) used a trans-rectal transducer to locate the uterine artery by lateralizing the transducer and using the external ileac artery as a reference point craniolateral to the bladder.
 
Measurements of umbilical artery
 
Doppler ultrasonography can be successfully utilized to assess the umbilical blood flow in foetuses of pregnant sheep and goats. The increase in umbilical PI and RI values after 19^th week of gestation may be indicative of decrease foetal growth rate and decreased blood flow and oxygen tension in foetal blood.  The size of the fetoplacental vascular bed and the Systolic/Diastolic ratio was related to umbilical blood flow and this link facilitated the detection of anomalies in size or function in foeto-placental vascularization, in addition to changes in the umbilical cord (Ramírez-González, 2023).
       
Serin et al., (2010), compared the blood flow of umbilical artery (UA) between singleton and multiple pregnancies of healthy pregnant Saanen goats and found no significant variations in umbilical artery with respect to pulsatility, or resistance in foetus. The RI values were more stable than the PI values. The 85^th and 130^th days of pregnancy showed two notable drops in the mean RI value, which was limited to 0.97 during pregnancy. A threshold stage for indices in the caprine pregnancy pattern was found to be the middle of the second trimester.
       
Doppler sonography was used by Kumar et al., (2015) to assess the hemodynamic features of the foetal umbilicus in Beetal goats from days 20 to 120 of gestation. Peak systolic velocity (PSV) increases significantly between days 39 and 67 and then again between days 98 and 120 of gestation (p<0.05); however, there was no discernible rise or fall in end-diastolic velocity (EDV). The characteristics of umbilical blood flow (UM) were examined by Elmetwally and Meinecke-Tillmann (2018) using 18 pregnant ewes and 20 pregnant goats. According to a qualitative assessment of UM blood flow, BFV, TAMV and TAMEAN levels in sheep and goat foetuses increased (P<0.001) until week 19, after which they dropped (P<0.001) until parturition. In both species, between weeks 4 and 12 of pregnancy, there was no end diastolic velocity (EDV) of umbilical artery blood flow.
       
According to Silva et al., (2022) the umbilical artery (UMB) in Saanen goats during physiological pregnancy was monitored using Doppler throughout the 21^st to 143^rd day of gestation. RIUMB demonstrated a low negative association with the number of foetuses per pregnancy and both PIUMB and RIUMB correlated with gestational age (p<0.001). The mean values decreased during pregnancy and the spectral trace’s flow pattern changed, although PSVUMB and EDVUMB values did not correlate with gestational age.
 
Measurements of uterine artery
 
According to Elmetwally et al., (2016), doppler sonography is a helpful technique for evaluating how maternal anxiety affects intrauterine foetal growth in pregnant goats and sheep. They also compared the groups that were more reactive (MR) and less reactive (LR). At weeks six, ten and twelve of pregnancy, UtA-PI was substantially greater in MR ewes than in LR ewes (P>0.01) and a tendency was noted at week fourteen (P=0.054). He concluded that ewes and goats have higher UtA-PI and UtA-RI in MR than in LR animals.
       
Beltrame et al., (2017), reported Santa Ines sheep’s doppler indices for uterine artery resistance, or the impedance of blood flow, decreased during the first, middle and late stages of gestation (PI: 1.15, 1.04, 31 0.97; RI: 0.61, 0.59, 0.57, respectively) (p<0.05). The hemodynamic indices for single and multiple pregnancies did not differ from one another, nor did the means of the variables in relation to the right and left uterine arteries. Sudha et al. (2019) evaluated the efficiency of Doppler indices for diagnosis of uterine torsion in Malabari does. The PI and RI values of arteries ipsilateral and contralateral to the side of uterine torsion measured the resistive index were 0.77 ± 0.03 and 0.56±0.03 and the values were observed to be higher in ipsilateral uterine arteries of torsion than in contralateral and normal kidding does; the value was lowest (0.47±0.02) in normal does.
       
Silva et al., (2022) assess the doppler sonography of pregnant Saanen goats’ uterine arteries blood flow. PSVUT, EDVUT, PIUT and RIUT, the mean values of the uterine artery doppler velocimetric variables, did not correlate with gestational age. The mean velocity of the uterine artery MVUT and PSVUT were negatively correlated with the number of foetuses per pregnancy. This suggests that the values of these variables will be lower when the pregnancy is multiple.
       
Akkus et al. (2023) assessed Awassi ewes to ascertain the alterations in uterine artery blood flow following the second half of pregnancy. Using Doppler ultrasound, measurements were made of the blood flow in the uterine arteries. The PSV value was found to be substantially greater on day 135 of pregnancy than it was on days 90, 105 and 120 (P<0.001). At day 135 of pregnancy, the EDV and TAP values were considerably greater than those days (P<0.01). On day 135, the RI value was considerably lower (P<0.001) and the PI and S/D values were significantly lower (P<0.01). They concluded that measuring changes in Doppler ultrasonography parameters will help to increase hemodynamic information because the most foetal development takes place in the second half of pregnancy.
 
Measurements of foetal aorta
 
Lee et al., (2005), the measurement of foetal aorta on 60, 75, 90, 105, 120 and 135 (cm) days of gestation is 0.22±0.001 to 0.57±0.07 in Korean Black goats. The regression equation for estimating gestational age by foetal aorta is y=151.1366x + 37.4774. The diameter of long and short axis of heart and foetal aorta were significantly increased with the gestational age (P<0.05).
               
According to Karadaev et al., (2018), the mean values of foetal aortic diameter was measured in Bulgarian local goats from 77, 91, 105, 119 and 133 days of gestation were 3.2 ±0.3 to 7.6 ±0.7, respectively. The results showed the high correlation between gestational age and the foetal aortic diameter, which could be measured between gestation days 63 and 133 (R² = 0.91). The regression equation of linear, quadratic and exponential associated between gestational age and foetal aorta in Bulgarian local goats were Y =10.97x+47.161, Y= -0.3996x2+15.218x+37.158 and Y=44.977x0.5481, respectively.

The ultrasonographic examination is performed using different techniques and it is an accurate diagnostic method for determining foetal number and estimating gestational age in small ruminants. Various foetal biometric factors are being effectively utilized to determine the stage of pregnancy. The foetal parameters, BPD, CRL, OSD, FTD, TD, Heart axis, FA, FL and KL are highly correlated with gestational age in small ruminants. During pregnancy, the uterine and umbilical blood flows rise continually to satisfy the increased nutritional demands of the growing foetus and placenta. Doppler indices UA, UMA and foetal aorta can be successfully evaluating the potential foetal risk during pregnancy and its prognosis.
 
Future prospects
 
It is high time to evaluate the breed specific echobiometric indices which has high correlation with gestational age in small ruminants. Doppler ultrasonography being a non- invasive method can safely be used to track changes in umbilical and uterine blood flow in normal and abnormal pregnancies. The application of 3D ultrasonography can improve the diagnostic confidence in veterinary practice by adopting standardized procedures.

The authors have no conflict of interest.