Risks of Pregnancies Too Close Together

Short interpregnancy intervals show up in the data as a risk marker, not as a verdict. Two 2019 systematic reviews limited to high-resource countries — the ones whose findings apply to a US reader — found real associations and were unusually candid about how shaky some of them are.

What goes up

For the baby, the reviewed evidence clusters below the six-month mark. Compared with longer intervals, an interpregnancy interval under 6 months was associated with increased risk of:

• preterm birth, the most consistently reported finding;
• small-for-gestational-age birth;
• infant death.

An earlier meta-analysis quoted in the same review put an interval under 6 months at roughly 40% higher odds of preterm birth, 61% higher odds of low birth weight and 26% higher odds of small-for-gestational-age birth. The Office on Women’s Health translates this for the public as a simple sentence: becoming pregnant again within a year of giving birth increases the chance the new baby is born too soon.

What goes down, which nobody mentions

The companion review on maternal outcomes found the opposite direction for two things. Short intervals came with lower risks of labor dystocia and of preeclampsia. What rose was precipitous labor, along with some evidence for pre-pregnancy obesity and gestational diabetes in the following pregnancy.

That mixture is worth sitting with. A risk profile that moves in both directions is not the profile of a straightforwardly dangerous thing.

Why the reviewers hedge

Both reviews and the HHS Office of Population Affairs work group land on the same methodological problem: reverse causation. A pregnancy that ends early — preterm, or in a loss — mechanically produces a shorter interval before the next one. So women with short intervals are partly women who already had a complication, and the interval gets blamed for it.

Add the fact that short intervals are not randomly distributed across income, age and access to care, and the honest summary is the one the reviewers wrote: some evidence, inconsistent results, and more research needed before the association can be read as cause. The three published spacing rules, and how far apart they are, are laid out in the pregnancy spacing calculator.

What is actually actionable

Two things, and neither is worry.

The first is the interval you can still choose. If a next pregnancy is a plan rather than a fact, the reliable lever is contraception in the postpartum months, and fertility comes back earlier than most people expect — see when fertility returns after birth.

The second is disclosure. If the pregnancy has already started with a short interval, the date of your last delivery belongs in the first prenatal visit, not in your head. It changes surveillance for preterm labor. After a cesarean it changes more than that, which is covered in how long after a C-section you can get pregnant.

This content is informational and does not replace prenatal care.