Pregnancy scanning

Pregnancy Scanning

Ultrasound scanning of mares during early pregnancy has become routine in recent years. This technique has greatly increased our knowledge of the changes that occur in early pregnancy, and enables veterinary intervention if problems arise.

Ovarian Follicle

Every foal starts life as an ovarian follicle. Mares release one (occasionally two) follicles every 21 days during the breeding season. The mare is a seasonally polyoestrus animal. This means that in response to the increasing day length in the spring the mare starts to come into season. She will cycle every 21 days until she becomes pregnant.

Horn of the Uterus

Near to ovulation the horn of the uterus prepares to accept the embryo under the influence of the hormone oestrogen. This causes an increased blood flow to the uterus, with oedematous (fluid filled) folds of endometrium (inner lining of the uterus) giving a ‘wagon wheel’ appearance in the ultrasound scan.


The follicle will ‘ovulate’ (release an embryo) from a particular place on the ovary called the ovulation fossa. This scan shows that the follicle is collapsing and that ovulation has occurred. The embryo now requires fertilisation by a stallion.

Post Ovulation Ovary

The mare has now ovulated. However the scan shows a problem – There appear to be two corpora lutea – this indicates that a twin pregnancy is possible.
Twinning should be avoided if at all possible in mares – as they are rarely carried to term and most often result in an abortion.

Twins day 12

A scan at day 12 confirms the suspicion that twinning has occurred. The picture on the far-left shows that the embryos are situated close together in the horn of the uterus. The picture on the right shows the same view with the embryos circled. The twins are carefully separated and the smaller of the two crushed. To reduce the pregnancy to a single foal.

15 Days

A scan 3 days latter confirms that the mare is still in foal and that only one embryo is present.

17 day scan

The pregnancy is usually located at the base of the uterine horn, and is spherical. Note the bright white lines at either side (top and bottom) of the vesicle – these are referred to as ‘polar caps’ – they are not part of the embryo, they are seen due to the fluid within the vesicle accentuating the ultrasound waves. At this stage the embryo cannot be seen. After day 16 the pregnancy starts to loose its circular shape.

22 day scan

At 23 days we begin to see the actual embryo that will develop into the foal. It is the small white bulge seen on the lower right hand side of the vesicle. At this stage the vesicle looses its spherical shape and is often described as being like a guitar pluck.

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