Let’s say you’d like to at least consider trying for a VBAC. What are some of the factors that influence your risk and likelihood of success?
Type of scar
The first thing you need to know is what kind of cut was made into your uterus during your earlier C-section: a classical (up-and-down) or transverse (side-to-side) incision. This is something you can only tell from your medical records. “It has nothing to do with the cut in the skin,” stresses Jon Barrett, chief of maternal-fetal medicine at Sunnybrook Health Sciences Centre in Toronto. Classical incisions, which are very uncommon, substantially increase the odds of rupture, so if you have this type of scar, most caregivers will recommend against a VBAC.
Number of previous Caesareans
While Society of Obstetricians and Gynaecologists of Canada (SOGC) guidelines state that women with two previous operations should be able to opt for a VBAC, this doesn’t always happen in the real world; some centres have a policy of not offering VBAC after two previous Caesareans.
How labour begins
IV medication (oxytocin) used to kick-start contractions, and hormonal gels and suppositories (prostaglandins) that are used to soften the cervix approximately double the chance the scar will separate. (By contrast, using a device called a Foley catheter to ripen the cervix has no such effect.) That’s why some centres rule out induction entirely if a woman is attempting a VBAC. Induction also decreases the chance that a trial of labour will end in a successful VBAC.
By contrast, going into labour spontaneously increases your odds of success. In fact, if your contractions start before a scheduled section, and you’re in advanced active labour by the time you get to hospital, not only are your chances of success increased, part of your risk of rupture has already passed, says Andrew Kotaska, clinical director of obstetrics and gynaecology at Stanton Territorial Hospital in Yellowknife.
Read more: The stages of labour>
Interval between births
Waiting less than 18 months after a C-section before attempting a VBAC may increase the odds of rupture. According to SOGC guidelines, this doesn’t contraindicate a trial of labour; still, some centres deny the option of a VBAC attempt less than 18 months after a Caesarean.
Reason for previous C-section
“The reason a woman had the Caesarean in the previous pregnancy can affect the chances of a successful VBAC,” Barrett observes, “though it doesn’t have as big an impact as people might think.” For instance, if your section was done for a reason that isn’t likely to recur, like the baby being in a breech position, “your chances of success are actually 80 percent,” notes Kotaska. On the other hand, if your previous labour stalled, your odds of success fall to about 60 percent.
Previous vaginal birth
Having given birth vaginally, either before or after your C-section, boosts your chances of success to roughly 90 percent.
Kotaska says there’s evidence women under 35 or who aren’t overweight may have better-than-average odds of a successful VBAC. Ditto for those whose babies are estimated to be smaller. However, estimates of fetal weight are notoriously inaccurate. Furthermore, adds Steele, it’s not uncommon for women who were told their C-section was necessary because the baby was too big to squeeze through the birth canal, to end up having a successful VBAC with an even larger baby.
Originally posted in July 2011.