VBAC Calculator

Predicting VBAC Success

VBAC scores are simple tools that account for several parameters to predict likelihood of a successful vaginal birth after prior caesarean section.

The Flamm model includes cervical assessment so can be administered following admission for labor whilst the Grobman model can be used before labor, when discussing birth planning with the mother.

A high VBAC score is predictive of greater chances of vaginal birth but a low VBAC score does not necessarily predict failure, so decisions should always be taken on a case by case basis, taking all circumstances in consideration.

Flamm Score Interpretation

In this study increasing score was associated linearly with increasing probability of vaginal birth after cesarean.

Grobman Score Interpretation

This scoring system is based on a nomogram, with scores ranging from 0 to 90, which account for 1% VBAC success chances to 99% VBAC success chances.

Some example scores and their success percentages are included below:

The Flamm VBAC score is based on a study on 5,022 patients, where multivariate logistic regression modelling was used to develop a predictive scoring system for vaginal birth, which was subsequently applied to a testing group to evaluate its predictive properties.

The Grobman VBAC score is based on a study on 766 women (with one prior low transverse cesarean who underwent a trial of labor at term with a vertex singleton gestation). The study looked at factors identifiable at the first prenatal visit and after development and analyzing the model with cross-validation techniques a graphic nomogram was published.

About Vaginal Birth After Cesarean

Firstly, it is important to define the two terms that you can come across with respect to vaginal birth after a previous caesarean section. TOLAC or trial of labor after caesarean section, describes patients who have agreed on trying a vaginal birth after caesarean whilst VBAC describes success cases.

Whilst the benefits and disadvantages of each of the two birth methods are widely known, attempting a VBAC may still be, in certain health settings, a rare occurrence as there are significant, yet rare complications, such as risk of uterine rupture or bleeding that may lead to hysterectory.

However, vaginal birth after a caesarean is considered to carry less complications than repetitive sections. Studies have shown a 60 to 80% success rate for women who attempt VBAC and this practice is recommended (where the mother is eligible) by the American Congress of Obstetricians and Gynecologists.

VBAC may not be recommended in the following cases:

• Type of incision – T shaped – carries a higher risk for uterine rupture;
• Labor dystocia – previous c-section due to abnormally slow or difficult labor;
• History – of multiple c-sections, especially after 3 or more;
• Health complications – if mother suffers from lung or heart disease;
• Fetal weight & gestational age greater than 40 weeks – especially if baby weight above 10 pounds.

References

Original reference

Flamm BL, Geiger AM. Vaginal birth after cesarean delivery: an admission scoring system. Obstet Gynecol. 1997; 90(6):907-10.

Grobman WA, et al Development of a nomogram for prediction of vaginal birth after cesarean delivery. Obstetrics and Gynecology. 2007; volume 109, pages 806-12, 2007.

Other references

Jeffries E, Falcone-Wharton A, Daggy J, Tucker Edmonds B. Examining the Impact of the Vaginal Birth After Cesarean Risk Calculator Estimation on Trial of Labor After Cesarean Counseling. MDM Policy Pract. 2019; 4(1):2381468319850830.

Dinsmoor MJ, Brock EL. Predicting failed trial of labor after primary cesarean delivery. Obstet Gynecol. 2004; 103(2):282-6.

Eden KB, Mcdonagh M, Denman MA, et al. New insights on vaginal birth after cesarean: can it be predicted? Obstet Gynecol. 2010;116(4):967-81.