Your pelvis has carried the weight of your baby for the last nine months – it’s that bony basin that cradles your baby as she grows in your womb. As you approach your due date, your pelvis will shift and move to allow your baby passage to the outside world.
A woman’s pelvis is designed for childbearing, and is made up of four bones – the paired innominate bones, the coccyx and the sacrum, which are held together by ligaments. It is divided into what is called true and false sections – the false pelvis (which is necessary to support your abdominal contents) and your true pelvis (which consists of the bony passage).
During pregnancy, the increased release of the hormone relaxin allows for softening and stretching to occur within the connective tissue of the pelvic joints. This results in a slight separation between the bones in the front of the pelvis, which then allows your baby’s head to move through. Your pelvis is smaller in relation to the size of your baby’s head, which means that during birth, your baby rotates as she moves across your pelvis in order to fit through. While the ligaments soften in preparation for birth, the unfused skull bones of your baby’s head gently mould over each other to enable her to negotiate the birth canal. It truly is an amazing act of nature.
Research by Caroline Homer, professor of midwifery, and Allison Cummins, lecturer and coordinator at the University of Technology in Sydney, Australia, reveals that your metabolism is essential in determining foetal growth, as well as providing support for labour and birth. “Starting pregnancy in the healthiest state and ensuring appropriate weight gain means you are more likely to start labour at the right time and deliver a baby who is a healthy weight.” They report that women who start pregnancy with a low or high pre-pregnancy body mass index (BMI) have an increased risk of preterm birth, small or large for gestational age babies, gestational hypertension, post-birth weight retention and more. Their research also shows that women with a high BMI are more likely to have their labour induced or deliver via C-section.
“It’s important to remember that most women are perfectly designed to grow a baby who is the right size for them to give birth to. As midwives, we have attended many small women who have given birth to normal or large-sized babies. Equally, we have also assisted larger women who have had difficulties giving birth to small babies.”
Judith Elaine Halek, director of Birth Balance and a childbirth educator, explains that the size and shape of your pelvic bones have an impact on how you look physically. Medically speaking, there are four basic pelvic shapes: gynecoid, anthropoid, android, and platypelloid.
This, says Elaine, is the classic female pelvis shape and is considered most favourable for delivery. It is also the most common shape. The pelvis is oval at the inlet, has a generous capacity and wide subpubic arch. “Women like this tend to be shapely and curvy. They tend to hold fat around the thighs more so than the midriff.”
Much like the gynecoid pelvis, this is oval at the inlet, but is wider from front to back than side to side. The subpubic arch may be slightly narrowed. This is the second most common pelvic shape. “Women with this pelvis shape tend to carry more adipose tissue or weight in their buttocks and abdomen.” With this pelvis shape, you may experience a slightly longer labour. It is recommended to take a more active role in labour − walking and squatting will be of benefit during the labour process.
This pelvis has a heart-shaped brim and is quite narrow in the front, creating a more triangular shape at the inlet and a narrowed subpubic arch. It is more likely to occur in tall women with narrow hips. Judith explains that larger babies may have difficulty moving through this pelvis. She adds that women with this pelvis shape tend to end up having C-sections depending on the size of the baby. Those who do give birth naturally may need to walk around more when in labour, and may need to push harder than those with other pelvic shapes.
This pelvis shape is flattened at the inlet and has a prominent sacrum. The subpubic arch is generally wide, but the ischial spines are prominent. This results in a kidney-shaped brim. The pelvic cavity is shallow and may be narrow. Judith adds that with this pelvic shape, you may find you carry your weight in the lower abdomen. Your baby may have a bit of difficulty entering the pelvis, but once she has passed through labour should run smoothly. This pelvic shape may mean your labour is slightly longer than those with other pelvic shapes, due to your baby taking longer to engage.
Ultimately, you will only know the true capacity of your body at the time of birth. However, knowing your pelvic shape, taking into account your baby’s size and speaking openly to your gynaecologist or midwife will help you prepare your birth plan and create the best possible birth experience.
Did you know?
A 2016 study, published in the Proceedings of the Natural Academy of Sciences suggests that the popularity of C-sections has had an impact on human evolution and, more importantly, pelvic size. Philipp Mitteroecker, a theoretical biologist at the University of Vienna and lead author of the study believes that an increasingly common reason women undergo a C-section is due to their pelvis being too narrow, and their babies too big, resulting in an “obstructed birth”. In the past (pre C-section) this condition often proved deadly. He says this could mean that genes for bigger babies are being passed down through generations, as C-sections have increased the number of live babies being birthed.