Are VBACs safe? Yes! Can you have a safe and successful VBAC with a previous diagnosis of a small pelvis or large baby? Yes!

We are hearing VBAC birth stories of more and more women being told that the reason for their primary C-section is because their pelvis is too small, or their baby is too big. 

As it’s based on bone structure, even plus size birth can elicit this diagnosis.

But, what does it really mean to have a small pelvis, and more importantly, how likely is it?

Originally published June 26, 2018, and was updated and republished on May 4, 2020.

What is a CPD pregnancy?

Cephalopelvic disproportion (CPD) is a term used for pregnancies in which the pelvic capacity is not big enough to allow the fetus to go through the birth canal. A reason for this complication may be a large fetus, a small pelvis, certain medical conditions, or a combination of other factors.

A pelvis too small for a baby is actually incredibly rare and very hard to diagnose. It is very discouraging for women and more often than not, leads to a woman having repeat c-sections for the rest of her babies without even being given a chance at vaginal birth.

The pelvis is able to mold during labor and, when laboring on positions other than your back, can expand by up to 30%!

If the operative report states a diagnosis of CPD, notice if they explain what position the baby was in at the time. More often than not, it is not a small pelvis, but that a baby was in a poor or less desirable position. 

A baby can enter the pelvis in many ways such as posterior, asynclitic, transverse, facial presentation, etc. If a baby engages in the pelvis in a poor position, it may be hard for them to turn and descend in the proper position so they can be born.  

How common is cephalopelvic disproportion?

Because CPD is so rare and hard to diagnose, it is hard to determine if a parent has CPD until something happens in the delivery. Even then, it may have been factors other than CPD causing difficulties in labor.

A provider may mention that it is dangerous to deliver a baby through a pelvis that has not been proven or has previously been diagnosed with CPD. During labor, fetal heart decelerations can cause concern, as the baby gets lower in the pelvis and has less room. A baby may show signs of stress when it feels squeezed in the pelvis in the second stage of labor. 

If a baby has heart decelerations, it could lead to cerebral palsy or even seizure after birth. In addition to a lack of oxygen to the baby, a provider may also be concerned about shoulder dystocia. 

Although these issues are extremely rare, they may be a concern and something to discuss with your health care provider to determine if vaginal birth is the right choice.

Diagnosing CPD: Is your pelvis really too small?

There sadly is not an exact percentage of how many people are diagnosed with TRUE CPD. It would be hard to diagnose someone by looking at them. 

However, there may be reason to believe that a person has true CPD if there has been a serious injury to the pelvis, such as a traumatic fall or accident. In some cases, childhood malnutrition or rickets can also cause true CPD. 

A provider may give a diagnosis of CPD if the following occurs:

• A diagnosis of a large baby.
• A previous diagnosis of a small pelvis, and a previous baby did not come out vaginally.
• The birthing parent has been diagnosed with diabetes.
• Failure to progress in labor: Labor is taking a long time, where the dilation and effacement is changing slowly with adequate contractions, OR the cervix is not making any change at all.
• Genetics, if a birthing parent’s mother has been told that their pelvis was too small, or they had cesareans. A provider may mention that it could be hereditary.  
• A baby is not descending past a certain point.
• The birthing parent is petite or short.

Diagnosing macrosomia aka “big baby”

A ‘big baby’ is actually defined by a baby who is more than 9lbs 15 oz.

Macrosomia, literally meaning big body, is when a baby is born weighing 11lbs or more! 16% of indications and 9% of c-sections are due to suspected big babies, when in reality, only 1.7% of babies are born bigger than 9lbs 15oz.

So what is really considered a “big baby”? 

According to Evidence Based Birth, it really depends, because researchers have different opinions. If a baby is anywhere between 8 lbs 13 oz and 9 lbs 15 oz, they may be considered macrosomic, “BIG.”

We enjoy the article all about CPD by Dr.M.D.Mazumdar, MD, defining everything in layman’s terms, and spelling out the truth about small pelvis and big babies:

Diagnosis of CPD is very difficult. This is because it is difficult to estimate exactly how much the mother’s ligaments and joints will ‘give’ or relax before labor starts.

The fetal head also has a great capacity to mold – the skull bones can overlap to some extent and decrease the diameter of the head. So, a baby who appears to be too big to pass through its mother’s birth passage before labour, may do so without much problem when active uterine contractions start.

A ‘trial of labour’ should always be given to all women with average-sized pelvis and an average-sized fetus even if the pelvis appears apparently too small for the baby.

Dr. MD Mazumdar

CPD Labor: Can you have a vaginal birth with a small pelvis?

Yes! It really is so hard to diagnose and know why exactly a baby does not seem to fit in the pelvis. 

I was told that I had CPD and my pelvis was “too small.” Providers said I would NEVER (and I say that with force because that is exactly how it was communicated to me), get a baby out of my pelvis. 

With my VBA2C baby, I had an amazing team that was trained and skilled in helping me get my baby in a better position, so he could come the way we had intended. Even with all our efforts, it took him a long time to flip from a posterior position, which was less ideal than an anterior position. 

Some babies, despite efforts to reposition, are still unable to change positions, leading to a diagnosis of CPD. 

Small people deliver big babies all the time. If you have been diagnosed with CPD it is important to look at all the factors before making a decision one way or another. 

Next steps to prepare for birth

If you have EVER been told your pelvis is too small to birth a baby or that you make babies too big for a vaginal birth, you NEED to read the article links above and have an educated conversation with your provider. 

We also suggest checking out Spinning Babies for their wonderful resources on helping create room in the pelvis, and getting a baby in a good position before labor begins. 

You may also want to plan, just in case, for the contingency that you will need a C-section. It is possible to have a family centered cesarean experience.

Did you know we have a podcast filled with VBAC stories? You may enjoy Episode 7, with a heartwarming story of a petite mom’s successful VBAC.

If you are working with a doula, it is important to communicate to them what happened with previous births, or what the provider may be saying. They can help the baby come down in the best possible way with different movements and supports. 

Planning a VBAC birth? “How to VBAC: The Ultimate Prep Course for Parents” will teach you everything you need to know to have a smooth, successful birthing experience.

Register today, and get ready to be strong, thriving, and knowledgeable when your new baby comes.