A previous birth predicts the timing of the next one weakly for term births and more strongly for preterm births. A previous labor that began at 41 weeks moves the expected timing of the next by about a day. A previous spontaneous birth before 35 weeks roughly triples the odds of another spontaneous preterm birth.
The labor calculator applies both findings, and the two effects are handled separately.
Term timing: a weak correlation
Ng and Steer (2016) examined the relationship between the gestational lengths of successive pregnancies in the same person. The correlation was 0.10: statistically significant in a large sample, but small. The length of a previous pregnancy explains about 1% of the variation in the next. Successive pregnancies also regress toward the population mean, so a very long or very short first pregnancy tends to be followed by a less extreme second one.
The calculator translates this into a shift in the term distribution's center of no more than a few days. In the baseline model, a previous spontaneous birth at 38 weeks moves the median of the next forecast to about day 280. A previous birth at 42 weeks moves it to about day 284. The two-day difference between first and later births reported by Smith (2001), 284 days versus 282, is applied separately and accounts for most of the change a second-time parent sees.
The shift is small because the evidence is weak. The evidence register records the coefficient as a conservative translation. The calculator does not apply it to a previous birth before 23 weeks, because the source study does not support that use.
Preterm birth: a stronger association
The relationship is different for preterm birth. Ananth and colleagues (2006) used linked records of consecutive singleton births. They reported an adjusted odds ratio of 3.6 for a spontaneous preterm birth before 35 weeks when the previous birth had also been spontaneous and before 35 weeks. Recurrence after a medically indicated preterm birth was weaker, with an adjusted odds ratio of 1.6.
Mercer and colleagues (1999) found that the gestational age of the previous preterm birth matters. The earlier the previous birth, the higher the chance of recurrence, and the earlier it tends to occur. Recurrence after a loss at 13 to 22 weeks was weaker and less consistent. Yamashita and colleagues (2015) reported an adjusted odds ratio of 2.26 for recurrence among women with two or more previous spontaneous preterm births.
The calculator applies these findings to its before-37-weeks component only. A previous spontaneous birth before 35 weeks raises the baseline 5% chance of labor before 37 weeks to about 14%. A previous spontaneous birth at 35 or 36 weeks raises it to about 10%. Two or more previous spontaneous preterm births add a further multiplier. In each case the applied coefficient is at or below the published estimate. The calculator also applies it to a wider before-37-weeks outcome, which is another reason the effect is stated conservatively.
The calculator also shifts part of the preterm probability toward the gestational age of the earliest previous preterm birth. Recurrence tends to occur near the same gestational age. A reported event at 20 to 22 weeks is recorded but receives no numerical adjustment, because the sources do not provide a stable coefficient for that range.
What the calculator asks
The form asks whether the user has given birth before, whether that labor began on its own, and at what gestational age. When there has been more than one spontaneous preterm birth, the earliest gestational age is used, consistent with Mercer's finding that the earliest previous birth is the most predictive. When there has been a medically recommended preterm birth and no spontaneous one, a smaller adjustment applies and is not combined with a spontaneous-preterm adjustment.
What the answer means for an individual
A history of term births moves the forecast by days. The calculator lists this shift on the result page as an adjustment. It is small. A second-time parent's window looks much like a first-time parent's, moved about two days earlier.
A history of spontaneous preterm birth is a different situation. The calculator reflects it as an increased before-37-weeks chance, but it is also a clinical matter with monitoring and treatment options that the calculator does not represent. Anyone with that history should be following their care team's plan, and the calculator's figure is not a substitute for it. The methodology page lists every adjustment and the study each is taken from.