A biological look at pregnancy

When a mare is pregnant, an entire world quietly forms inside her uterus. Long before you see a growing belly or feel the excitement of foaling season approach, complex biological systems are already working together to support new life.

Understanding how a foal develops inside the mare helps breeders feel calmer, more confident, and better prepared. Instead of guessing what’s happening beneath the surface, you gain clarity about the biological processes that support new life from the very beginning.

Let’s take a closer look at how the foal grows and is supported inside the uterus, from a biological perspective.

The Mare’s Uterus: Built for Pregnancy

The mare’s uterus has a Y-shaped structure, consisting of a main body and two long uterine horns. After conception, the embryo initially develops at the base of one of these horns.

As pregnancy progresses and the foal grows, it expands beyond the horn and into the main body of the uterus. Eventually, both the main body and one horn support the developing foal.

This structure is one reason twin pregnancies are problematic in horses; the uterus is designed to support one foal very well, rather than two moderately well.

The Placenta: How the Foal Is Nourished During Pregnancy

Unlike in many other mammals, the mare’s placenta covers almost the entire inner surface of the uterus. This widespread attachment allows for efficient exchange of nutrients, oxygen, and waste products between mare and foal.

In early pregnancy, the outer foetal membrane (the chorion) is relatively smooth. Around day 70, it begins to change dramatically. Thousands of tiny, finger-like structures called villi develop, giving the placenta a soft, velvety texture.

These villi interlock closely with the uterine lining, creating a very large surface area. This design allows nutrients from the mare’s bloodstream to be absorbed efficiently and delivered to the foal. It’s an elegant, highly specialised system — and one that relies heavily on good maternal health.

The Foetal Membranes and Fluids

Inside the uterus, the foal is protected by several layers and fluids, each with an important role.

The Amnion

The amnion is the inner membrane that directly surrounds the foal. It contains amniotic fluid, which cushions the foal, allows movement, and protects against physical shocks. At full term, this fluid amounts to roughly 3.5 litres (6 pints).

This is the white membrane that you may see first during delivery.

The Allantois

Outside the amnion lies the allantois, which contains allantoic fluid, approximately 8.5 litres (15 pints) at full term. This is the fluid released when the mare “breaks her waters” at the start of labour.

Together, the chorion and allantois form the chorioallantoic membrane, which makes up the placenta.

The Umbilical Cord: Connection and Exchange

The foal is connected to the placenta via the umbilical cord, which contains:

• One vein

• Two arteries

• The urachus (a tube that removes waste from the foal’s bladder during pregnancy)

Through this cord, oxygen and nutrients flow to the foal, while waste products are carried away. Until birth, this connection is the foal’s entire life-support system.

Foetal Positioning Inside the Uterus

Throughout pregnancy, the foal is carried in what might seem like an unusual position, essentially upside down, with its belly facing the mare’s spine. This is completely normal.

Only during the first stage of labour does the foal rotate into the correct position for birth. This rotation is a natural and crucial part of the foaling process, and it’s one of the reasons that early labour is such an important phase.

The Cervical Star: A Special Landmark

There is one small but significant area of the uterus where the placenta does not attach — the point closest to the cervix. This area is known as the cervical star, named for its distinctive star-like appearance.

This attachment-free zone plays an important role during foaling and is a normal anatomical feature. Knowing about it can help understand placental structure after birth.

Why Twin Pregnancies Are High Risk

Twin pregnancies do occur, but they are rare and usually problematic. Because the placenta is designed to nourish one foal using the full uterine surface, twins must compete for space and nutrients. Typically:

• One foetus grows faster and dominates

• The smaller foetus gradually becomes deprived of nourishment

• This often leads to abortion, loss of one foetus, or non-viable offspring

For this reason, early ultrasound scanning (around 15–25 days post-ovulation) is essential. When detected early, twin pregnancies can often be reduced to a single, viable pregnancy — greatly improving the outcome.

What This Means for You as a Breeder

This biological system is incredibly efficient, but also delicate. It relies on:

• Good maternal nutrition

• Low stress

• Consistent care

• Early veterinary involvement

Understanding how the foal develops inside the mare helps explain why these factors matter so much. It also reminds us that pregnancy is an active, complex collaboration between mare and foal.

When you support the mare well, you support everything happening inside her.

A gentle reminder

You don’t need to memorise anatomy or master every detail. But understanding the big picture gives you calm confidence.

And that’s one of the greatest tools you can bring into your breeding journey.

Understanding foetal development also makes it easier to follow the broader breeding process step by step, which I outline in The Breeding Journey Workbook. Click here to download it today and start exploring your possibilities.