Modern research on the accuracy of gestational age calculations for goats is scarce, but one paper from 2011 compares three different methods in Saanen goats.
In this research by Abdelghafar et al. (2011), crown-rump length was measured at the 5th, 6th, 7th, 8th, 9th and 10th weeks of gestation. Measurement markers were placed at the upper part of the skull, to the end of the sacrum (end of the spine), when the fetus was fully extended. In the event that the fetus was curved, measurements were taken in two phases; head to heart, and heart to sacrum. The method used for CRL is shown on the second page of this paper: http://www.idosi.org/gv/gv6(4)11/1.pdf
Biparietal diameter (BPD) was measured weekly from week 6 to the end of gestation, in coronal section, making sure the head was as oval in shape as possible, with the mid line dividing the two hemispheres clearly visible. The authors did point out that in late gestation, during 3rd trimester, this method became increasingly challenging. The head was often compressed by surrounding parts, and the reduction in the amount of uterine fluid in late pregnancy made finding the head in the first place very difficult.
The authors also measured umbilical cord diameter, but did not find this to be as accurate as BPD or CRL.
It is worth remembering that gestational length in this study was found to be 146 plus or minus 12 days, so if your predicted due date is within this tolerance, it may very well have been accurate. Also remember that your specific breed of goat may have slight differences in growth rate and gestation time to published results, which you will learn to compensate for as you begin to keep records of your findings.
- BPD is not accurate before 40 days in sheep, and was not validated before 6 weeks in goats in the present study. After this point, however, BPD was found to have the greatest accuracy in gestational age estimation. It has also previously demonstrated accuracy with Nubians, Toggenburg, Pygmy and Angora goats.
- CRL was also found to be very accurate in this study, as well as in previous work on Nubians and Egyptian Balady goats.
- It is important to note that the authors specified a strict measurement protocol, and followed it, every single time. This is a major principle underpinning all accurate use of ultrasound; repetition of a consistent method, and a commitment to obtain the best images, every single time. Most professional bodies ask for evidence of a minimum of 200 scans as proof of competency in performing a particular measurement technique. Responsible hospitals and veterinary practices will perform regular checks of intra- and inter-operator variability in measurements. If you are working alone, it is up to you to maintain your standards and ensure reproducibility.
Finally, remember to look for other clues. Does the 2D image fit your measurements? A heartbeat should be visible from 20 days; limb buds visible around the 30 day mark; nostrils and eyes from day 42.