FACTORS TO CONSIDER
Cause of previous Caesarean
This can be divided into two categories:
- Recurrent indications: these are factors that are likely to affect future deliveries, such as poor labour progression, cephalopelvic disproportion, prolonged second stage, failed induction or macrosomia.
- Non-recurrent indications: these are factors that relate solely to the previous delivery, such as foetal distress, malpositioning (transverse/breech), severe preeclampsia, placenta previa or placental abruption.
You are more likely to have a successful VBAC delivery if your previous Caesarean was performed due to a non-recurrent indication, with rates of success similar to those of women delivering for the first time. For example, if you are trying for VBAC following a Caesarean for breech presentation, you have a projected success rate of 89%. In comparison, VBAC success rates after prior Caesarean for CPD/failure to progress range between 50-67%.
- Women with a recurrent indication for their previous Caesarean are statistically less likely to have a successful trial of labour (TOL) to those with a non-recurrent indication.
- If you previously had a Caesarean due to twin pregnancy, and had a single low horizontal incision, you are still a candidate for TOL.
- If your prior Caesarean was due to a prolonged second stage labour or shoulder dystocia, you may still be a candidate for TOL but it is worth discussing with your midwife or Obstetrician as to how long you trial labour.
Previous incision type
There are statistically better outcomes for low horizontal Caesarean incisions compared to that of vertical or T-shaped uterine incisions, as these are associated with a higher risk of uterine rupture.
History of previous successful Vaginal Delivery prior to Caesarean
Prior Vaginal delivery, including prior successful VBAC, is the strongest predictor of a successful VBAC. The success rate is increased when women had a prior VBAC (93%) compared to a Vaginal delivery prior to Caesarean delivery (85%).
If you have a successful VBAC, your risk of uterine rupture reduces and stays reduced for all subsequent Vaginal deliveries.
History of multiple Caesarean deliveries
A history of multiple Caesarean deliveries, as well as other major abdominal surgery that breaches the uterine cavity (such as laparoscopic myomectomy) does increase your risk of uterine rupture. However, it may still be possible to have a TOL if you have a history of two low horizontal incisional surgeries. There have been several studies that show similar rates of VBAC success with two previous Caesarean deliveries (VBAC success rate of 62-75%) compared to single prior Caesarean delivery. However, >50% of the multiple-Caesarean women within these studies had also had a previous vaginal birth and 40% had had a previous VBAC, and we know from the above information that these factors can improve VBAC success.
Foetal weight after Caesarean
Baby birth weight of 4kg or more is associated with a higher risk of uterine rupture, unsuccessful VBAC, shoulder dystocia, and larger degree perineal tears. If you are trying for VBAC and have no history of prior successful vaginal delivery, your chance for successful VBAC is less than 50%.
It is difficult to give an approximate birth weight based on ultrasound sonography within the third trimester, so while we encourage you to consider it as a potential risk (especially if your previous babies have been above 4kg consistently), it may not preclude you from VBAC. In fact, 60-70% of women who attempt VBAC with a macrosomic foetus are successful.
Your body and genetics can affect your VBAC outcome. Women below the age of 35 were more successful and had fewer complications during a VBAC. If you are caucasian, your VBAC success rate is higher (78%) than non-caucasian populations (70%)
Maternal weight can also impact VBAC success. Women of average BMI were found to have a 70.5% VBAC success rate, compared to overweight (65.5%) and obese (54.6%) BMI individuals. If you were to gain weight after being a normal BMI within your first pregnancy, your rate of successful VBAC decreases significantly compared to women who maintained a normal BMI. Alternatively, if you were to successfully lose weight to achieve normal BMI before your second pregnancy this unfortunately does not increase your rate of successful VBAC.
Time between deliveries
An interpregnancy interval refers to the months between a woman’s last delivery and onset of amenorrhea within the next pregnancy. A short interpregnancy interval (<2yrs) is associated with a higher risk uterine rupture, which again increases if the interval is <18 months.
Gestation and Labour Presentation
While we can try to predict your risk with all of the factors above, it may also come down to how you present at the time of labour. An ideal presentation would be a spontaneous labour at either term or preterm, with an anterior occiput position after rupture of membranes. Oxytocin augmentation has been found to have a lower rate of successful VBAC rather than any intervention and may be associated with uterine rupture.
If you are admitted with cervical diameter greater than 3 cm, you have a stronger likelihood of vaginal delivery compared to those admitted with less than or equal to 3 cm (i.e. latent first stage of labour). Presence of meconium-stained liquor and labour stay lasting more than four hours after admission were associated with high failure rate of VBAC.
The bottom line
We can’t tell you whether a VBAC will be your best option – the most we can do is make you aware of any factors within your presentation that may adjust your chance of successful delivery. The choice is ultimately yours – but we are here to help you express your concerns, educate you with the most up-to-date information available and help you feel comfortable and confident in your decision.
Written by Liz Johnson, Women’s Health Physiotherapist
Female Physio Co.
References: Flamm BL, Newman LA, Thomas SJ et al. Vaginal birth after Caesarean delivery: results of a 5-year multicenter collaborative study. Obstet Gynecol 1990; 76 (5 Pt I): 750-4. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7233729/pdf/ACTA-90-300.pdf https://utswmed.org/medblog/vbac/#:~:text=Vaginal%20birth%20after%20cesarean%20section%20(VBAC)%20often%20is%20an%20option,had%20a%20prior%20C%2Dsection. ]