With so much to look forward to as the New Year begins, and foals taking their first wobbly steps all over social media, we felt it was an excellent time to look at how these extraordinary creatures come into being.
From day zero
Any discussion about the development of a foal has to begin on day zero, the moment of fertilisation. Sperm is deposited into the uterus through a live covering by a stallion or artificial insemination, and the sperm travel to meet an egg, or ovum, in the fallopian tube of the mare. The joining of the egg and one sperm is referred to as fertilisation.
Once fertilisation has taken place, cell division starts to occur. This cell division occurs at a pretty set rate. The first division happens after 48 hours, resulting in four to six cells. The second occurs after 72 hours, resulting in eight to ten cells. Between days four and five, the mass of cells present is termed a morula. The day after the morula has been formed, it starts to differentiate into a mass of one type of cell on the interior and a layer of another type of cell on the outside. At this point, it is referred to as a blastocyst.
Around this time, the blastocyst moves from the fallopian tube of the mare into the uterus, and from the following week, the number of cells in the blastocyst starts to increase rapidly. Due to the movement of the uterus, the blastocyst migrates around the uterine surface and makes contact with all areas. This process is crucial for the mare’s body to recognise her pregnancy and also ensures that she maintains the pregnancy and does not come into season.
Pregnancy check
The first pregnancy check by the vet usually takes place between days 14 and 16. At this point, the blastocyst is easily recognised on ultrasound scan and is about 2cm in size. Beginning on day 17, the embryo will undergo the process of fixation to the uterine wall, so scanning in advance of this is highly recommended to allow twin pregnancies to be detected and managed by the veterinarian; once the embryos are fixed, it is much more difficult to remove one of the embryos without causing damage or risking losing both embryos simultaneously.
Twin pregnancies
Twin pregnancies are highly undesirable in horses. In two-thirds of cases, abortion will occur naturally, allowing only one of the two embryos to survive. However, in some cases, both embryos will develop into viable foetuses. If abortion occurs early in the process, the owner may not notice anything other than that the mare is no longer pregnant. However, if abortion occurs later, owners may be faced with two large stillborn foals. In rare instances, fully-developed twins will be born live, but few of these survive.
Little life
Around day 17, we see more embryonic development. Firstly, the neural tube appears, which will become the foal’s brain, spinal cord and nervous system. Then, blood vessels form at the base of the spine, and these become the heart. A heartbeat can be detected on ultrasound somewhere between days 21 and 23. Around day 37, the umbilical cord is formed in the amnion. For most people, this marks the end of the embryonic stage, and the developing foal is now termed a foetus.
A 40-day-old foetus is easily recognisable as a miniature foal with a tiny head, legs, hooves and body. At this stage, the foetus measures around 5cm. All organs are present and developing into their final form. The main job of the developing foal at this point is to grow!
At around 150 days of gestation, the foal weighs approximately 2kg and is the size of a small cat. At this stage, the foal has developed eyelashes and hair around the muzzle (cute!). At around day 180, the foal’s weight just about quadruples to approximately 8kg and it is the size of a small dog. The mane and tail have begun to grow. By day 240 the foetus weighs around 16kg and by day 270 it weighs approximately 30kg and is completely covered in hair.
Final stages
As the foal approaches day 300 of gestation, the lungs undergo their final development in preparation for life outside the womb. At around day 315, you should start to prepare for foaling and monitor the mare closely. A gestation range of 320 to 365 days is considered normal, but it is best to be prepared! At delivery, if all goes according to plan, the foal should shift from lying on his back to being born with his head and forelegs first.
And just like that, we welcome a little foal into the world!