Developmental Stages & Qualitative Estimation of Gestational Age

With experience, it is possible to estimate gestational age by eye. Developmental stages in goats are less well-documented than in many other species. The below landmarks are more or less universally agreed. Days are counted from day of mating:

• Heartbeats can be seen from day 24 with high-resolution scanners, and seen easily by the majority of scanners from day 35 (Suguna et al. 2008).
• By day 42, the head of the embryo can be clearly identified.
• By day 55, the skull, spinal cord, rib cage, forelimbs and hind limbs can be seen (Kumar et al., 2015).
• By day 76, internal organs can be seen, such as the kidneys, liver, bladder and stomach. The heart should be very easy to identify at this point.
• By day 82, some authors report being able to identify genital tubercles, and the scrotum in male fetuses.

Number counting is also easiest between days 45-55, where the pregnancy is still early enough that individual gestation sacs can be seen on screen. As the kid(s) begin to grow, it can be difficult to discern one from another, as they can often become superimposed upon one another. If trying to estimate numbers later in pregnancy, the best method is to look for individual heartbeats.

Placentomes

Placentomes can be identified from day 38, but authors who have correlated placentome diameters with gestational age have done so from day 50. When measuring placentomes, the published method is to choose the three largest on screen, measure the diameters using your ultrasound’s ‘caliper’ function, and average your results. At day 50, the average diameter is around 1cm, increasing to just over 3cm (3.07±0.2cm) by day 130 (Suguna et al., 2008). There is clearly a strong positive correlation between placentome diameter and biparietal diameter (head diameter), but reference values for different goat breeds are currently absent from the literature, meaning that, at the present time, observations of the size of cotyledons is qualitative (or semi-quantitative). There is nothing to stop you from becoming the first person to establish reference values for your chosen breed!

Above: Cotyledons in a very advanced pregnancy (the leg bone of the kid can also be seen stretching across the screen), measuring over 3cm in diameter.

Above: Cotyledons in a less advanced pregnancy. The largest is around 2cm in diameter.

Heart beats

The stage at which you can detect a heartbeat is highly dependent upon the resolution of your ultrasound transducer, and the optimization of your controls. You should reliably be able to see the flicker of a heartbeat from day 35, but may see it sooner. Depth, frequency and focal zone adjustments are particularly important here. A high quality ultrasound machine utilizing electronic beamforming is capable of detecting the flicker of a heartbeat at least 5 days earlier than a mechanical sector system.

It is good practice to save a cine loop of the beating hearts that you identify, as it demonstrates due diligence.

Above: Heartbeat captured on a Scan Pad.

Heart rate

The heart rate of the kid or kids decreases significantly as pregnancy progresses, giving another semi-quantitative indicator of gestational age, but again, reference values are lacking. The only way to calculate the fetal heart rate without Doppler is with M-mode.

M-mode actually preceded B-mode (Brightness Mode, the 2D scanning mode that you are familiar with), and was the most advanced ultrasound technology available in echocardiography (cardiac ultrasound) up until the 1970s. M-mode uses a single scan line, and this one-dimensional line is plotted against time. It can be useful for tracking fast-moving structures, like the canine foetal heart, which normally beats at over 220 beats per minute (bpm).

You can calculate the heart rate from the M-mode trace, but it is important to refer to your ultrasound machine’s manual (or call your supplier) before doing so, because it varies between manufacturers on whether to use one or two cardiac cycles. One clue that you should be using two cycles when you have used only one is that you will end up with a calculated heart rate of over 400bpm!

The only normal values currently available in goats are 162.2±1.5beats/min during early pregnancy, to 130.8±3.6beats/min during late pregnancy. The way I remember it by is 130bpm at 130 days, ~35bpm more than this from 35 days, but in reality, I’ve never used heart rate for any other reason than personal interest and a bit of fun. There are much more accurate ways – both qualitative and quantitative – for estimating gestational age.

In the canine world, researchers have shown that monitoring the unborn puppies’ heart rates with M-mode can alert the user to fetal distress. This is potentially of greater use than using it for estimating gestational age. However, it’s important to note that they observed some level of distress at some point in one third of pregnancies, so this should be used as a supporting measurement only, not in isolation as a cause of unnecessary panic.

Above: M-mode performed on a Scan Pad. The M-mode trace shows the changes in the position of structures over time.

References:

Kumar, K., Chandolia, R., Kumar, S., Pal, M., Sandeep, K. (2015). Two-dimensional and three-dimensional ultrasonography for pregnancy diagnosis and antenatal fetal development in Beetal goats. Vet World, 8(7):835-840.

Suguna, K., Mehrota, S., Agarwal, S. et al. (2008). Early pregnancy diagnosis and embryonic and fetal development using real time B mode ultrasound in goats. Small Ruminant Research, 80, 80-86.