When your period comes back, and when your fertility does
Ovulation can come before the first period, and the advice about spacing is weaker than it sounds.
Ovulation can come first. In a systematic review of women who were not breastfeeding, the mean day of first ovulation ranged from day 45 to day 94, and between 20% and 71% of first periods were preceded by an ovulation. The conclusion: Most nonlactating women will not ovulate until 6 weeks postpartum. A small number of women will ovulate earlier, potentially putting them at risk for pregnancy sooner, although the fertility of these early ovulations is not well-established.
Waiting for a period as the signal to start contraception is therefore too late for a meaningful minority of women.
pregnancy is unlikely if menses have not returned, infant is less than 6 months old, and infant is fully or nearly fully breastfeeding with no interval of more than 4-6 hours between breastfeeding sessions.
Used exactly that way it is about 98% effective, which is roughly two pregnancies per 100 women over six months. But it expires abruptly: at six months, at the first period, at the first bottle, and at the first night the baby sleeps through. In the United States, where 24.9% of babies are exclusively breastfed at six months, the number of women who actually meet all three criteria at that point is small.
On spacing, the honest version is more complicated than the advice. ACOG says: Women should be advised to avoid interpregnancy intervals shorter than 6 months and should be counseled about the risks and benefits of repeat pregnancy sooner than 18 months.
The traditional evidence behind that is a meta-analysis of 67 studies finding that intervals under 6 months carried a 1.40 times higher odds of preterm birth. Then two large studies compared each woman against herself across her own pregnancies:
| Preterm birth, interval under 6 months | Compared across women | Compared within the same woman |
|---|---|---|
| Australian cohort, 40,441 mothers | 1.41 (1.31 to 1.51) | 1.07 (0.86 to 1.34) |
| Canadian cohort, 38,178 women | 1.53 (1.35 to 1.73) | 0.85 (0.71 to 1.02) |
In the Canadian cohort the absolute figures were 12.8% preterm with a short interval against 8.2% with an 18 to 23 month interval, and in the within-woman analysis that difference disappeared. The authors: Previously reported associations between short interpregnancy intervals and adverse neonatal outcomes may not be causal.
So the best argument for spacing is not that a short gap will make your next baby premature. It is your own recovery: iron stores, weight, blood sugar, pelvic floor, and mental health. That is a better reason and a truer one.