Is giving birth more complex for Humans?

Is Giving Birth harder for humans than other mammals?

We know that giving birth is more complicated for humans than it is for other mammals. To understand why, it is important to understand the differences in pelvis shapes and sizes. There are distinct differences between human female and male pelvi (pelvi: plural of pelvis) and pelvi of the great Apes.

The anatomy of the female and male human pelvi.

Being Born: Human babies compared to ape babies.

For the human baby to be born it must make a series of turns and movements through the mothers pelvis. These movements are known as the Cardinal movements (CMs). CMs are in response to the baby being pushed down through the pelvis by the contraction of the uterus.

Cardinal movements: moving through the maternal pelvis to birth.

As pressure increases on parts of its head and shoulders, the baby must turn to fit through the mother’s pelvis. The baby’s own reflexes, coupled with the downward force from contractions help to steer it through the available space following the ‘path of least resistance’.

The cardinal movements of the baby occurs in the order of: 

  • Engagement: When the top of baby’s head moves down into the pelvis. This can happen in late pregnancy with first babies but often not until labor begins with second or subsequent babies.
  • Descent: When the baby’s head moves deeper into the pelvis as it is pushed down by the strength of contractions. This moving down happens throughout labor.
  • Flexion: The baby’s head tips forward, and the chin touches it’s chest. This happens in early labor.
  • Internal Rotation: This allows the longest part of the baby’s head to match the longest part of the mothers pelvis
  • Extension: The baby’s chin comes off its chest and the neck arches as the head is born. The baby’s face is now looking towards the mothers anus.
  • Restitution/External rotation: After the head is born, there is generally a short pause, particularly in 1st births. This allows the baby to now turn again to fit it’s shoulders through the widest part of the mothers pelvis. Once this happens the baby’s face will move from looking down to now facing either of the inner thighs. This movement allows the shoulders to fit through and under the pubic arch at the front of the mothers pelvis.
  • Expulsion: after the shoulder closest to the pubic arch moves out from under the it, the opposite shoulder follows shortly. The rest of the baby’s body is then born. Occasionally this process happens all in one apparent movement relatively quickly once the head is born.

Head moulding: making a big head smaller to fit through the pelvis        

Another adaptation that takes place is the moulding of the baby’s head to make it smaller. It is very common for newborn’s to have what appears to be an oblong head shape immediately following birth. This allows the baby to fit through the maternal pelvis and happens during labor as baby is pushed down through the birth canal.

What is moulding?

Head moulding during labor is caused by the movement of the bones of the baby’s scull in response to downward pressure from contractions and resistance from below from the mothers tissues and pelvis shape. These forces make the bones of the baby’s scull move to change the shape and size of the baby’s head to make it easier for it to be born. The bones of the baby’s scull can easily move and even overlap because they are not yet fused together. The ability of the baby to adapt the size and shape of its head to fit through the maternal pelvis is more noticeable in some babies than in others. It usually changes back to the more expected round shape within 24 hours following birth.

Fetal head molding
Moulding of the baby’s head at birth and after 24 hours

Cardinal movements and head molding are necessary for human babies because of the shape and size of the baby’s head compared to the size and shape of the human female pelvis it must move through to be born. It is when one of these movements does not happen, for any reason, that babies become ‘stuck’, and labor does not progress.

The pelvis shape and size of humans compared to the great Apes

However, in other mammals, the size and shape of their pelvis, compared to the size and shape of their babies head is very different to humans. Specifically in the great apes that are genetically the closest to humans; these cardinal movements don’t, and do not need to happen.

Relative sizes and shapes of fetal heads to maternal pelvi compared to humans

The pelvi in great apes is longer and narrower than the human female pelvis, and their babies heads significantly smaller.

How does the Position for Labor and for Giving Birth make a difference?

As we have seen there is much less space for human babies to be born through their moms pelvis than there is for baby apes! This is why it is really important for women to maximise every little bit of space within their pelvis when they are giving birth by moving freely and instinctively and more upright birthing positions.

Upright positions for labor include standing, kneeling, squatting, all fours, lunging or sitting upright.