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The Logistics of Childbirth: The Fetal Skull and Parental Pelvis

I remember thinking to myself many a time with my first born "how is the baby going to come out of me"? While most of us are familiar with the concept that the baby travels down and out the vaginal canal, it leaves a lot of people still pondering the logistics of how this happens. Today, let's dive a bit deeper into how the baby is able to fit through the pelvis in order to be born.

Photo Credit: Hui, J. (n.d.). Joyce Hui | portfolio. Joyce Huiart. Retrieved February 27, 2021, from

The relationship between the fetal skull and the parent pelvis is symbiotic. Both are able to adapt during labor and delivery to aid towards a positive outcome.

Over the course of the pregnancy, thanks to a hormone concoction referred to as relaxin, our ligaments have softened and loosened. Our pelvis bone structure is held together by these ligaments which means that when the ligaments soften, it allows the pelvis to have some wiggle room. The pelvis begins to be able to flex at the joints where normally they are firm and rigid. When it comes time to start the birthing process, labor begins and progresses and the baby enters lower into the pelvis. As this takes place, the pubic symphysis in the anterior part of the pelvis is willing to open a bit to allow more room for safe passage. On the posterior side of the pelvis, the sacrum is willing to move as well in order to give space as necessary to let the baby down and out.

While the parent pelvis is busy transforming and adapting, so is the fetal skull.

The baby’s skull is not fully fused, which means that it can adapt in shape to a certain extent. This is for the intention of navigating through the birth canal. The birth canal is not a direct shot out into the world. There is some curvature to it which means that the baby has to shift positioning a bit throughout the delivery process. Sometimes labor slows at certain points and depending on where your baby is positioned at the time of the slowing, the fontanelles of their skull will begin to push closer to each other in order to take up less surface area. This phenomena is called fetal skull moulding.

Between fetal skull moulding and the shifts that take place in the pelvis, your baby is allowed down and out of the uterus and into the world.

Happy birthday baby!


  1. Young, K. A., Wise, J. A., DeSaix, P., Kruse, D. H., Poe, B., Johnson, E., Johnson, J. E., Korol, O., Betts, G. J., & Womble, M. (2013). Anatomy and Physiology by OpenStax (hardcover version, full color) (1st ed.). XanEdu Publishing Inc.

  2. Dietz, H. P., & Bennett, M. J. (2003). The Effect of Childbirth on Pelvic Organ Mobility. Obstetrical & Gynecological Survey, 58(11), 722–723.

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