VBAC Success Rates


If you want to learn about the reasons for a VBAC, read Why try a VBAC?

As you read, VBAC success rates are high, but according to the Society of Obstetricians and Gynecologists of Canada (SOGC), many factors can positively or negatively impact the outcome. You first have to evaluate all the conditions and elements before making recommendations.

The biggest VBAC question that needs to be asked is why did you have a caesarean section for your past childbirth(s)? If the principal reason for the surgery were a recurrent cause, such as a narrow pelvis, obstructed labour with an incompetent cervix, a congenital problem, etc., the situation wouldn’t change with the arrival of another baby. You are who you are. Your underlying weaknesses and health problems will not change from one day to another. However, many causes of a past caesarean section will not necessarily be present during another childbirth. For example, breech birth (the baby’s buttocks first rather than their head), the baby’s heart showing signs of fatigue, transversal positioning of the baby, praevia placenta, etc.

Here are a few factors that make us think that VBAC can be successful. Then I will talk about other risk factors that need to be evaluated before trying.

Positive Factors for a Successful VBAC:

  • If you have already given birth vaginally
  • If you have already started labour (effacement and dilation of the cervix)
  • If the cause of your previous caesarean is not recurrent (ex.: breech birth)
  • If labour started on its own, naturally (induction is possible but not the best option)
  • If the uterus was opened horizontally during the previous caesarean (90-95% of cases)
  • If you have had a successful VBAC in the past
  • If the mother is under 40

During emergency caesareans or other special situations, or in some medical practices in other countries, the uterus may be opened vertically. This method increases the risk of uterine rupture during a future pregnancy and increases the risk of trying a VBAC.

Factors to Consider before Trying a VBAC

According to the SOGC, these factors are not themselves contraindications for a VBAC. Instead, they are essential elements your doctor will consider based on your medical history before suggesting a VBAC.

  • The mother’s age (risks increase after 35, and even more so after 40)
  • If the cause of the previous caesarean is recurrent
  • If there was difficulty removing the baby’s shoulders during an earlier vaginal birth (shoulder dystocia)
  • Several previous caesareans (tissue is fragile) or other abdominal surgery
  • If there are less than 18-24 months between two caesareans
  • If the scar site on the uterus is unknown, such as for immigrant women who do not have medical files, or if the suture direction is not ideal for VBACs
  • A macrosomic baby (a very big baby over 4kg). In medical terms, this is called cephalopelvic disproportion. This means that the mother’s pelvis (pelvic cavity) did not have enough space for the baby’s head (cephalo) to pass, which is the largest part of the baby that needs to pass through a mother’s pelvis
  • If she is post-term (after 40 weeks). The baby keeps growing (macrosomia)
  • Induction (or stimulation), provoking contractions (or increasing them with stimulation) using synthetic hormones. It’s not impossible for a woman who had a previous caesarean, but the dosages need to be low and given prudently and diligently. It depends on many factors, including the changes in the mother’s cervix, as induction causes risks for uterine rupture. You have to know the maternity ward’s policies and how your doctor works. This procedure is seen as controversial by the medical community
  • If she is obese
  • If she has other related complications, such as hypertension, diabetes, preeclampsia, placenta accreta, etc.

A vaginal birth will not occur after a previous caesarean if:

  • There was a previous uterine rupture
  • Surgery on the uterus other than a caesarean
  • A previous caesarean scar is vertical (depending on the doctor and procedure, as this situation increases the risk of uterine rupture by 10-12%)
  • And other conditions associated with the mother or baby (ex.: breech birth)

To continue reading, go to How to Soothe Pain during a VBAC.

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