Accurate early pregnancy diagnosis can sometimes be a challenge particularly if you have not previously examined the mare. Cysts may be mistaken for an early pregnancy or twin. Detailed breeding history and thorough scanning technique is vital to avoid potentially costly mistakes.

Cysts:
Uterine cysts are fluid filled, immobile, anechoic structures. They can occur anywhere in the uterus but often are located at the base of either uterine horn which may increase confusion with diagnosis of single or multiple pregnancies. Cysts may be single or multiple; intra or extra-luminal; regular or irregular in shape; large or small and may be compartmentalised. Cysts often have a thicker, hyperechoic wall compared to a vesicle.
Various studies have found that cysts are more common in older (>10 years), multiparous mares. Prevalence of cysts in one study (Eilts et al. 1995) was approximately 27% of 295 mares examined during one breeding season. Mares >11 years were 4.2 times more likely to have endometrial cysts than mares <11 years.

Twinning:
Twinning rate varies between breeds. There is a high incidence in Thoroughbreds for example and low incidence in ponies. Certain mares twin repeatedly and highly fertile stallions tend to have a higher incidence of twins. Ovulation induction agents may also increase twinning rate (Veronesi et al. 2003) Twins are almost always due to double ovulations and are just as likely to originate from synchronous or asynchronous ovulations. Double ovualtions may occur on the same ovary or opposite ovaries so always check both ovaries at every scan. Triplets are rare but can and do occur!
Monozygotic twins have been reported but are rare. There may be a higher incidence with embryo transfer. A case report involving embryo transfer of a single vesicle highlights importance of multiple pregnancy examinations to detect twins (Mancill et al. 2011).

Breeding history:
Obtain as much of the previous breeding history of the mare as possible. This includes age, breed, number of foals, date of last covering, ovulation date, previous history of twinning or multiple ovulations and whether or not ovulation induction drugs have been utilised. Information from previous reproductive ultrasonographic examinations, such as uterine mapping of cysts and detection of multiple ovulations can be very helpful.

Ultrasonographic evaluation of early pregnancy:

Initial considerations:
Ensure adequate scanning conditions such as good equipment, lighting and mare restraint. If possible the mare should be examined before covering to identify any cysts. They may change from year to year following pregnancy but do not usually change within the current breeding season. Mapping of size and location of cysts can be extremely helpful.
Ideally the first pregnancy scan should be performed at Day
14 - 15 post ovulation. If twins are present both vesicles should be visible on careful ultrasound examination. Multiple ultrasound examinations are essential to monitor growth of vesicle and to detect the embryo and heartbeat. Doppler ultrasound may be useful for detection of heartbeat.
Always examine both ovaries as there will usually be one corpus luteum per pregnancy apart from the very rare occurrence of monozygous twins.

Early embryonic development:
By 10 days post ovulation the pregnancy is visible ultrasonograhically as a 4 - 6 mm diameter, spherical, anechoic vesicle. It is highly mobile and continues to grow 1.5 mm/day until Day 12.
By Day 12 the vesicle is approximately 8 mm diameter and is still spherical and highly mobile. There are usually 2 hyperechoic short lines on dorsal and ventral borders of the conceptus known as specular reflections. These are not an indicator of pregnancy but a physical phenomenon caused by the reflection of the ultrasound beam. Cysts by comparison tend cause nonspecular reflections.
Between Days 12 - 16 the vesicle grows at around 3.5 mm/day and is still spherical in shape and highly mobile. By Day 14 the vesicle is approximately 14 - 18 mm.
Fixation of the vesicle at the uterine horn-body junction occurs by Day 16. The vesicle is regular but slightly ovoid in shape. By Day 21 the vesicle can become quite irregular and may be confused with intraluminal fluid or cysts. The uterus may have a slight oedema pattern due to normal follicular development or oestrogen production by the pregnancy.
Embryo detection may occur as early as Day 21 in the 5 or 6 o'clock position but should be reliably detected by Day 24 when it is ventrally located.
Heartbeat may be detected as a rapid flicking movement in the centre of the embryo as early as Day 23 however should definitely be detectable by Day 26 when the embryo is approximately 8 mm in length in the 7 o'clock position.
By Day 30 the embryo has grown further and divides the pregnancy into 2 halves which can appear like twins if this is the first scan so it is important to identify the heartbeat (one or two?). By Day 32 the embryo is approximately 12 mm in length and usually located in the dorsal part of the vesicle at 12 o'clock position.
Ideally identification and management of twins should be performed prior to development of endometrial cups.