Obstetricians and expectant parents often note that first-time births are more likely to be delayed, attributing this to the body’s initial physiological adjustment to labor. While some literature suggests first births may be more prone to late deliveries,1 other studies have found first births to be more prone to a preterm delivery.2 This study aims to quantify how birth order affects the likelihood of a “late” delivery, helping expectant mothers and their clinicians better understand what is typical for first births compared to subsequent ones.
To understand the relationship between birth order and “late” delivery, we studied more than 2 million pregnancies resulting in a live, full-term, single birth between 37- and 43-weeks’ gestation, excluding inductions or cesarean deliveries. We compared gestational duration and rate of delivery beyond 41 weeks between those who have had a prior delivery and those who have not.
When evaluating the rate of deliveries taking place past 41 weeks’ gestation, we found that 6.2% of first-time deliveries among mothers with no prior pregnancy extended beyond this point. This was 1.53 times more often than for those with a prior delivery, where only 4.0% delivered after 41 weeks. Among mothers with a prior pregnancy loss before 20 weeks, 5.8% of first full-term births occurred after 41 weeks.
When looking at the distribution of gestational age at delivery, we found that patients who have not had a prior delivery have an average gestation of 275.9 days compared to 274.5 days among mothers who have delivered before, a statistically significant difference, as seen in Figure 1.
