A 3D illustration of foal foetus at day 65

It’s very exciting to be involved in breeding a live, healthy foal. Besides, who doesn’t love the sound of tiny hooves cantering around in the paddock alongside your beautiful mare? In part one, which appeared in the July issue (HQ124), we explored the mare’s reproductive cycle. We now look at the stages of gestation of the pregnant mare.

Falling pregnant

Pregnancy can occur when an egg has been fertilised in the mare. Fertilisation of the egg is necessary for an embryo to form, which constitutes the first form of life for the unborn foal. Fertilisation occurs in the oviducts, also known as the fallopian tubes. Sperm is deposited into the uterus (either by the stallion’s live covering or by artificial insemination). Once that occurs, sperm travels to meet with an ovulated egg, or ovum. This process can take approximately six to seven days.

The uterus houses the embryo, which in turn becomes a foetus. The uterus is designed to house, nourish and protect the growing foetus. The mare’s uterus possesses two unique characteristics in terms of reproduction in domestic animals. First, the mare’s uterus possesses an amazing ability to defend itself against massive bacterial contamination – in particular, the ability to significantly reduce endometrial inflammation (inflammation in the uterus caused by infection). Second, placental development and function begin relatively late in gestation in the horse, which means that uterine gland secretions must support the embryo for a large part of the first trimester.

Little life

During the early days of gestation, the embryo is restless, moving throughout the uterine lumen from the tip of the horn where it entered, to the body of the uterus, and into the other horn. The embryo moves around the most between days 12 and 14 of gestation. On about day 16, the embryo stops moving (migrating) and settles at the base of one of the two uterine horns. On average, this is usually on day 15 for ponies, and between days 15 and 17 for horses. The embryo increases in size from 3 to 4mm in diameter per day between days 17 and 24. A heartbeat can be detected by about day 21.

Between days 22 and 24, the allantois, an embryonic membrane, is formed. On days 21 through 40, the allantoic sac becomes larger and the previously large yolk sac disappears as the umbilical cord develops. From day 40, the foetus will be enveloped by the amnion (the membrane that lines the chorion) and amniotic fluid. The chorion is the outermost placental membrane.

By 45 days of gestation, the attachment of the foetus-containing sac to the uterus approaches an advanced stage. Little projections or villi of foetal origin interlock with the endometrial crypts. This interlocking action forms the microplacentomes, which is the tissue that functions as a unit of exchange for pregnancy. The microplacentomes continue their support and development of the pregnancy for the next four months.
At around 150 days of gestation, the foal weighs approximately 2kg. At this stage the foal has developed eyelashes and hair around the muzzle. At around day 180, the foal’s weight just about quadruples to approximately 10kg. Mane and tail hair have begun to grow at this stage. Relatively speaking, there is actually minimal growth prior to this point.

Final stages

By 250 days of gestation, the mare will show noticeable weight gain around her belly as the foal grows, gaining as much as a kilogram per day. The majority of foetal growth occurs in the final three months of gestation. This means, in those final months as the foetus is rapidly growing, the mare’s nutritional demands will increase. As the foal approaches day 300 of gestation, the lungs are developing in preparation for life outside of the womb.
The mare’s udder swells and expands and begins to produce a sticky yellow discharge that will turn into milk approximately two weeks prior to the birth of the foal. The mare’s belly grows heavier, with her vulva relaxing and lengthening as the time for foaling approaches.
At around day 315, you should be prepared for foaling and monitor the mare closely on a daily basis. The size of the foal at birth is often determined more by the mare’s uterine capacity than by genetics, although genetics do play a role once the foal has been born. Once the foal is born, genetics and nourishment play a big part in growth and development. A foal who has the genetics for a large size can grow rapidly in the first few months of life.

The full article appears in the September issue (126) of HQ > Shop now