Hot temperatures could be linked to early birth, scientists say.
A study found a spike in birth rates when temperatures reach 32.2C when researchers looked at birth records of 56 million Americans born between 1969 and 1988.
During this period, an average of 25,000 babies were born up to two weeks early each year as a result of heatwaves, according to the study.
This represents a loss of more than 150,000 gestational days annually, according to scientists from the University of California Los Angeles (UCLA).
Shorter gestational periods have been linked to negative health and cognitive outcomes later in life.
Previous research has linked hot weather to an acceleration of childbirth and shorter gestations.
Scientists used estimated changes in daily birth rates from counties across America to quantify the total number of lost days of gestation associated with heat over a span of two decades.
The sample for the research published in Nature Climate Change included 56 million births.
Researchers estimate that birth rates increase by five per cent on days with a maximum temperature above 32.2C, with an average gestational reduction of 6.1 days - some births occurred two weeks early.
The authors, Dr Alan Barreca from UCLA, and Dr Jessamyn Schaller of the National Bureau of Economic Research, in Massachusetts, write: "We find that extreme heat causes an increase in deliveries on the day of exposure and on the following day and show that the additional births were accelerated by up to two weeks.
"While we posit climate change will cause gestational losses, the exact magnitude of the future costs is highly uncertain - households may adapt as expectations about the frequency of hot weather events increase, which could mitigate impacts on infant health."
Andrew Shennan, professor of obstetrics at King's College London, said: "The findings are valid and well researched but they do not take into account why women deliver.
"This requires further evaluation to understand mechanisms.
"Extremes of temperatures, both hot and cold have previously been linked to risk of both stillbirth and delivery. Mechanisms are not clear.
"Given the wide variety of temperatures around the world, and that most women have normal pregnancies, this is unlikely to be an important risk factor for any individual."
Australian Associated Press