How Does A Large Head Fit Through A Small Exit?

How Does A Large Head Fit Through A Small Exit?

It’s the question most pregnant women have pondered at some point.

How on earth does a baby’s head fit through such a small exit?

After hearing all the comparisons between a baby’s head and a large fruit, they might have concerns about how the physics will work during labour and birth.

Although it can be hard to imagine, especially if you haven’t given birth before, in almost all cases, a baby’s head is perfectly able to fit through the birth canal.

In the rare situation where the pelvis is too small for the baby, there is usually a known reason, such as severe injury, or congenital abnormalities.

So if you’re worried about how your baby’s head is going to fit, read on. Find out why a c-section doesn’t have to be your only option.

What Happens During Labour?

During labour, your baby’s head will pass through a gap in the pelvic bones before it reaches the vaginal opening. At this point, the size of the baby’s head is less important than the position the baby is in.

A baby in the occiput position (back facing the mother’s belly) is in the optimal position for birth. As the baby descends through the pelvic outlet, the smallest part of the head presents first. This makes the descent through the pelvis much easier.

A baby in another position might not present the smallest part of the head first. Some babies are in posterior position (back facing the mother’s back) or have their heads flexed, rather than tucked in. This means the largest part of the head is leading the way.

Women who are free to move about in labour tend to adopt positions that help the pelvis remain ‘open’ and flexible. A woman who is lying on her back, or is semi-reclined on a bed, immobilises her pelvis. This reduces the amount of space the baby has to move through.

How Your Baby Helps

We often think of babies as being the ‘passengers’ in the journey of labour and birth. But they are not simply  ‘waiting’ to be born; they participate actively.

Babies can wriggle into a good position for birth. They twist and turn, and sometimes they push with their feet. They can also fall asleep! But most importantly, their heads are amazingly designed to fit through the birth canal.

A newborn baby’s skull isn’t a fixed bony structure. It is divided into several plates. These plates are separated by soft areas, which allow the plates to move and overlap as the head is squeezed through the pelvis and vagina.

So essentially, your baby’s head can adjust to fit the size of your body.

What Happens To My Body?

During pregnancy, your body releases a hormone called relaxin. As its name suggests, this hormone encourages the ligaments in your body to relax and loosen, in preparation for giving birth.

The pelvis is not a fixed bony structure, either. There is a joint at the front called the symphysis pubis which connects the two halves of the pelvis. This joint is made up of fibrocartilage – a tough and very strong tissue, found in ligaments and tendons.

During birth, this joint is flexible and allows more space for the baby to move through the pelvis. If you are able to squat, kneel with legs wide apart, or assume a lunge position, you allow the baby even more space in the pelvis.

As your baby moves down through the dilated cervix, the bones in his skull will shift to fit through. As this is the widest part of the baby, it leads the way for the rest of the baby’s body.

At the same time, thanks to another hormone called estrogen, the tissues of the vagina will begin to stretch. This increases blood flow to the vagina, so the connective tissues can expand during the second stage, when your baby is descending.

Most women are surprised (and often disappointed) to feel their baby’s head come down, and then go back up during the pushing stage. While this can cause frustration, it is perfectly designed to allow your body to stretch gently.

As the baby’s head begins to move further down, the perineum starts to stretch. This is the area of skin and connective tissue found between the vagina and anus. The perineum is capable of stretching to allow the baby’s head through, especially if the mother is in a free position (not lying down), and relaxed (tight muscles can hinder this stretching).

When women push instinctively with each contraction, the perineum is more likely to stretch gently, and without tearing. It will unfold and stretch until it becomes paper thin. This can be a very intense sensation, and many women describe it as a burning or stinging feeling. Warm water can help the tissues to stretch with little pain, and warm compresses can relieve any pain.

What Does My Care Provider Have To Do With It?

When it’s a question of a large head fitting through a small exit, choosing a care provider who supports normal birth can make all the difference.

If the person who supports you during pregnancy and birth has faith in you, you are more likely to feel confident and  prepared to work with your body during labour.

A care provider who has restrictive policies about movement, positions, time limits, and so on will undermine your ability to let your labour unfold. You will be look for external motivators rather than just listening to your body and working with your baby.

Doubt is powerful. It can get in the way of your belief that your body is perfectly designed to create a baby, and to give birth to that baby. Choose your birth support team wisely, with that in mind.

Recommended Reading:

Small Pelvis? Big Baby? Here’s The Truth About CPD

Active Birth – 8 Big Benefits For Mother and Baby

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Sam McCulloch Dip CBEd CONTRIBUTOR

Sam McCulloch enjoyed talking so much about birth she decided to become a birth educator and doula, supporting parents in making informed choices about their birth experience. In her spare time she writes novels. She is mother to three beautiful little humans.


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