The study, which analysed 148,500 Canadian pregnancies in the space of a decade, found a gap of less than 12 months was the most dangerous – increasing the risk of maternal death, premature birth, stillbirth and low birth weight and adviced couple to Wait until a year before new pregnancy
Study leader Dr Laura Schummers, whose findings are published in the JAMA Internal Medicine journal, said: ‘Our study found increased risks to both mother and infant when pregnancies are closely spaced, including for women older than 35.
‘The findings for older women are particularly important, as older women tend to more closely space their pregnancies and often do so intentionally.’
The researchers found the risks were different according to a woman’s age.
For those over 35, a short gap between pregnancies was particularly dangerous for the mother herself.
For younger women, the risk to the mother of a short gap between pregnancies was negligible, but there was an increased risk of the baby being born premature.
Among women over 35 who conceived six months after a previous birth, the researchers found a 1.2 per cent risk of maternal death or severe harm.
If they waited 18 months between pregnancies, the risk was more than halved to 0.5 per cent.
For younger women, a six-month gap between pregnancies resulted in an 8.5 per cent risk of preterm birth – delivery before 37 weeks – which dropped to 3.7 per cent with an 18-month gap.
Among over-35s, the preterm birth risk is six per cent for a six-month gap and 3.4 per cent for an 18-month wait.
Fellow researcher Dr Wendy Norman, of the University of British Columbia, said the findings provide the first ‘excellent evidence’ to help women guide the spacing of their children.
‘Achieving that optimal one-year interval should be doable for many women, and is clearly worthwhile to reduce complication risks,’ she said.
Scientists believe a short gap between pregnancies does not leave the body enough time to recover.
On the other hand, leaving it too long brings the risk of complications associated with an ageing mother.
Professor Sonia Hernandez-Diaz, of the Harvard T.H. Chan School of Public Health, added:
‘Whether the elevated risks are due to our bodies not having time to recover if we conceive soon after delivering or to factors associated with unplanned pregnancies, like inadequate prenatal care, the recommendation might be the same – improve access to postpartum contraception, or abstain from unprotected sexual intercourse with a male partner following a birth.’
Experts from the University of Colorado, writing a linked editorial piece in the same journal, said women increasingly feel under pressure to leave shorter gaps between pregnancies, because so many are having their first child at an older age.
‘Because the mean desired number of children US women is 2.6 and fertility declines rapidly in the late 30s and early 40s, women having a child when they are older than 35 years are likely to plan to have their next child soon thereafter,’ they wrote.