Ultrasound
In the modern veterinary practice, ultrasonography is the most common method used to diagnose pregnancy in the mare as it is can routinely be used to detect pregnancy as early as day 12 - 14 following ovulation.
The ultrasound works by emitting high frequency sound waves. As the sound waves move through the tissue, some are reflected back to the transducer, converted to electrical current and displayed on the screen. The number of waves reflected is proportional to the density of the tissue. As sound waves pass through fluid, few waves are reflected and the screen appears black. As the sound waves pass through tissue, waves are reflected back and the screen appears gray.
The frequency of the sound waves generated by the transducer determines how far the sound waves will penetrate into the tissue. Lower frequency transducers (3 and 3.5 MHz) are used to image structures like the lungs, heart or kidney. High frequency transducers (5 to 7.5 MHz) are used to examine the reproductive tract.
The safety of ultrasound has been documented with mares palpated or scanned on days 15, 20, 25, 30, 35, 40 and 50 days. There was no difference in the pregnancy rates of mares palpated, scanned with the machine off or scanned with the machine on (57, 53 and 67%).
The animation further explains how the ultrasound works.
During ultrasonography of the mare to detect pregnancy we will be looking for the embryonic vessicle. This is a black sphere seen on the ultrasound screen and represents the fluid that is within the early embryo and latter the fluid contained within the developing placenta that surrounds the embryo and/or fetus. Below you can see what ultrasound scans of a day 12 and 19 mare looks like. Note how the shape of the vessicle changes.
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Actual Ultrasound Scan
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View Ultrasound Scan
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The general diameter of the embryonic vessicle on different days of early pregnancy are shown in the table:
| Day of gestation | 10 |
20 |
25 |
30 |
35 |
40 |
50 |
| Mean size (cm) | 1.96 |
2.73 |
3.22 |
3.62 |
4.42 |
5.94 |
8.84 |
| ± | 0.50 |
0.36 |
0.31 |
0.27 |
0.12 |
0.16 |
0.11 |
In addition to viewing the uterus to determine the presence or absence of an embryonic vessicle it is also possible to access the uterus for presence of edema which would indicate the mare is about to lose the pregnancy and return to esturs. This could be further checked by examining the ovaries for the absence of a large corpus luteum and the presence of a large follice likely between 25 to 30 mm in diameter.
