Are more babies born in low pressure?

Midwives have said it for generations. There are actually studies. They're both better and worse than you think.

Midwives have told each other on the ward for generations: more babies are born when air pressure falls. It sounds like the kind of claim no one has ever bothered to check. They have.

The labour onsets that followed the pressure drop

In a 1997 American study, the number of labour onsets was counted against air pressure over time. There were more labour onsets in the 24 hours following a marked drop in pressure.

Waters break — but contractions wait

Another study found something similar, and something different, at the same time: significantly more births and more instances of waters breaking at low air pressure. But when they looked specifically at when the contractions themselves started, there was no significant association with air pressure. Birth as a whole was linked to the weather. The starting gun wasn't.

Direction doesn't matter

A 2022 Japanese study found yet another angle on the matter. It wasn't whether pressure fell or rose that mattered. It was how much it changed. On days with a bigger pressure change — regardless of direction — the number of births rose, and the association was statistically significant.

That's roughly the same pattern found in parts of the headache literature: some people react when pressure falls. Others react when it rises. If you only measure direction, half the signal can disappear into the average.

The criticism that looms largest

But the work that perhaps looms largest in this field isn't yet another finding. It's a critique of the others. A 2020 review points out that the literature on weather and births generally suffers from too few patients, inadequate weather data and insufficient control groups. Other studies — which have asked the same question — have simply found no association.

Yet another American study, from 1996, asked the same question about pressure changes and labour onset in women at term — one more attempt among several looking for the same answer without arriving at one everyone can agree on.

That makes births and low pressure a fine example of something that recurs in almost all research on weather and the body: everyone has a story. No one has a clean answer.