You may have heard from your provider that you want your baby to be “vertex”, or positioned head-down in your pelvis for the delivery. However, head-down status is not the only consideration. There are, in fact, many varieties of the vertex position, and tons of factors contributing to how your labor and birth will proceed. This two-part series will explore:
- varieties in fetal position
- the parts of your anatomy, and how they can affect your baby’s position or the progress of your labor
- healthy body mechanics for pregnancy and how to encourage optimal fetal position
- techniques to correct issues
The baby’s inability to achieve optimal fetal positioning due to outside factors, such as the uterus/uterine ligaments, pelvis, and supporting musculature, is known as intrauterine constraint.
Your baby’s position in the uterus is named by 1) the presenting part, or which part of your baby is entering pelvis first and 2)whether that part is facing the front, back, or side of your body.
About 96% of all babies will go vertex by 36 weeks, due to the weight of their head.
Some head down babies’ heads may be asynclitic, with their head tilted towards one shoulder, or posterior, with their spine towards the mother’s back. These positions, while not ruling out the option of a vaginal birth, are not ideal and can make for challenging labor patterns, a potentially more difficult second stage, and an increased likelihood of operative delivery.
Let’s review some essential uterine and pelvic anatomy during pregnancy.
The Bony Pelvis
The pelvis is not one bone, but four: the two innominate bones (you can feel the crests of these at the top of each hip), the sacrum, and the coccyx, or your tailbone. The symphysis pubis, the hard spot you feel at the base of your abdomen under your mons pubis, is commonly referred to as the “pubic bone” but is not actually a separate bone. It is the cartilaginous joint of the two innominate bones of the pelvis. There is a similar joint on either side of the sacrum, connecting all three of these bones together to create the pelvic bowl. These joints are called the sacroiliac (SI) joints, and are a common location for aches and pains during pregnancy when weak or misaligned. Finally, a 4th joint attaches the coccyx to the base of the sacrum. These areas of cartilage are somewhat flexible, and the hormones of pregnancy increase this flexibility.
When the SI joints are misaligned, the shape of the pelvic bowl may be altered and can limit the baby’s ability to descend into the pelvis or rotate adequately, leading to asyncliticism or posterior lie.
The bones of the pelvis come together to create the passage through which your baby will pass during delivery. The shape of this passage varies and is classified into one of four pelvic types. 50% of Causcasian women have a Gynecoid pelvis and nearly half of African American women have an Anthropoid pelvis (a shape also associated with a higher frequency of Occiput Posterior fetal position).
“The variety of shapes, combined with the variety of fetal head presentations, plus size variations, mean that labors vary greatly.” -Spinningbabies.com
The Ligaments and Fascia
Round ligaments: attach to pubic bone and top of uterus
Broad ligament (not pictured): connects sides of the uterus to the walls and floor of the pelvis
Uterosacral ligaments: attach the posterior side of the cervical uterus to the sacrum.
These ligaments suspend the growing uterus in the abdomen. If any of these ligaments is tight or torqued, the uterus may be altered in shape and restrict the baby’s ability to freely move. Asymmetry of the ligaments can torque the lower uterine segment and change the alignment of the cervix, making dilation more difficult and painful.
Fascia is the membrane that wraps around all of the body’s muscles and organs and can be damaged by trauma, scar tissue development from surgery, or from poor body mechanics over time, leading to obstruction of movement.
Many of your muscles can contribute to your stability and well-being in pregnancy. Some of these may also affect your baby’s position.
The psoas, or ‘hip flexor,’ is a posterior abdominal muscle that helps to stabilize the pelvis. It attaches at the thoracic vertebrae and wraps around, over the pelvis, and attaches at the top of the thighs, creating support for our abdominal organs. During pregnancy, the psoas helps support the growing uterus and assist with core stability. The psoas should be stretchy and mobile, and when tight can encourage pelvic misalignments and prevent fetal descent in labor. Prolonged sitting, postural imbalances, cycling, and running may all contribute to the tightening of this muscle.
Alternately, excessively lax abdominal tone ( which may be result of previous pregnancies, see our post on diastasis recti) can lead to a pendulous abdomen, where the baby extends too far outward from the mother’s body. The result is a fetal head that is not settled directly into the pelvis and applied to the cervix, potentially impeding labor progress.
To learn more about the various pelvic types, intrauterine constraint, and optimal fetal positioning , visit spinningbabies.com or attend one of their spectacular workshops.
Stay tuned for part two of this article, where we discuss what you can do to correct these problems and encourage optimal fetal positioning for labor and birth!