Women who have their babies less than a year apart are twice as likely to give birth prematurely, a new study suggests.
Around one in five mothers whose babies are born less than 12 months apart will give birth to their babies before they reach 37 weeks of pregnancy - also classed as a pre-term birth.
This compared to just 7.7% of mothers who wait for the "optimal" time of 18 months or more between children.
Babies born prematurely can suffer a range of problems - generally the earlier a baby is born, the higher the risk of health issues.
The US study, published in BJOG: An International Journal of Obstetrics and Gynaecology, examined 450,000 births from women who had two or more pregnancies during a six-year period in Ohio.
They found that mothers with "inter-pregnancy intervals" (IPI) of less than 18 months were more likely to give birth prior to 39 weeks of pregnancy than those who waited longer between children.
More than half of those who had an IPI of less than 12 months had their babies before 39 weeks compared with 37.5% of those who had an "optimal IPI", the authors said.
They said that women should be counselled on the importance of "optimal birth space".
"Short inter-pregnancy interval is a known risk factor for pre-term birth, however this new research shows that inadequate birth spacing is associated with shorter overall pregnancy duration," said study co-author Emily DeFranco, assistant professor of maternal fetal medicine at the University of Cincinnati College of Medicine in Ohio.
"This study has a potential clinical impact on reducing the overall rate of pre-term birth across the world through counselling women on the importance of adequate birth spacing, especially focusing on women know to be at inherently high risk for pre-term birth."
Commenting on the research, Louise Silverton, director for midwifery at the Royal College of Midwives, said: "This study supports advice that midwives give to women about birth spacing. If women are to breastfeed for the recommended six months before introducing solid foods, they may delay ovulation and assist in birth spacing.
"Women need access to contraceptive advice to allow them to space their births. In the UK specialist family planning service provision is patchy with GPs frequently offering only oral contraception. Specialist services should be available for all women."