How we calculate labor probabilities
We use a mixture of two skewed normal distributions to model the probability of spontaneous labor onset. This approach captures both preterm deliveries (before 37 weeks) and term deliveries more accurately than a single distribution.
The mixture model combines:
• Preterm component (~11% weight): Models early deliveries with a center around 36 weeks
• Term component (~89% weight): Models the majority of deliveries, peaking just after the due date
Our model parameters are fitted to match official maternity statistics:
The fitted model achieves the following statistics:
10.3%
Preterm rate
Day +0.2
Median delivery
48.6%
Deliver by due date
All probabilities shown are conditional on not having delivered yet. This means we recalculate based on the fact that you're still pregnant. For example, if you're at 39 weeks, we only consider the remaining probability distribution from 39 weeks onward.
P(labor on day X | still pregnant) = P(labor on day X) / P(labor after today)Research shows several factors can shift the expected delivery date by a few days. We apply these as location adjustments to the distribution:
Parity
First-time mothers tend to deliver ~2 days later
Bergsjø et al. (2001) - BJOG
Fetal Sex
Boys arrive ~1 day later on average (280.6 vs 279.8 days)
Divon et al. (2002) - Am J Obstet Gynecol
BMI
Higher BMI associated with later delivery (up to +5 days)
Denison et al. (2008) - BJOG
Interpregnancy
Short intervals (<12 months) associated with earlier delivery
Shachar et al. (2017) - Obstet Gynecol
These probabilities are for spontaneous labor onset only. They do not account for:
• Scheduled inductions or cesarean sections
• Medical conditions that may affect timing
• Individual variation beyond the modeled factors
• Research shows individual prediction has limited accuracy (c-statistic ~0.56)
Key research informing this model:
1.
Bergsjø P, et al. (2001)
Duration of human singleton pregnancy. BJOG
2.
Divon MY, et al. (2002)
Male gender prolongs pregnancy. Am J Obstet Gynecol
3.
Denison FC, et al. (2008)
Maternal BMI and timing of spontaneous labor. BJOG
4.
Shachar BZ, et al. (2017)
Interpregnancy interval and obstetric outcomes. Obstet Gynecol
5.
NHS Digital (2024)
NHS Maternity Statistics, England 2023-24
6.
CDC NCHS (2024)
Births: Final Data for 2023. NVSR Vol 73 No 1
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