Too short or long a gap between children increases the risk of problems for the second baby, research suggests.
An interval of less than 18 months and more than 59 months - just under five years - increases the risk, the Colombian researchers found.
The team looked at 67 previous studies covering over 11m pregnancies.
Writing in the Journal of the American Medical Association, said it was best to leave between two and five years between pregnancies.
Risk assessment
The team from the Fundacion Santa Fe de Bogota analysed research carried out across the world between 1966 and 2006.
"Women and couples will take this paper into account when making their decision"
The studies all took the age of the mother and her socioeconomic background - which can affect pregnancy outcomes - into account.
The researchers compared the women with the smallest gaps, of less than six months, with those who left a "mid-length" gap of between 18 and 23 months between pregnancies.
The pregnancies with a shorter gap had a 40% increased risk of premature birth, a 61% increased risk of low birth weight and a 26% increased risk of being small for their gestational age compared to those with a longer gap.
Infants of mothers with pregnancy intervals longer than 59 months had a 20% to 43% greater risk of adverse outcomes.
For each month that time between pregnancies was shortened from 18 months -
For each month beyond 59 months -
Motivation
The researchers, led by Dr Agustin Conde-Agudelo, said the risks associated with short intervals between pregnancies might be explained by the mother's body not having enough time to recover from the physiological demands of having the first baby and breastfeeding.
The effect of a very long gap could be a gradual decline in the capacity of a woman's body to bear the demands of pregnancy - similar to the state seen in women in their first pregnancy.
The outcomes for babies born a long time after their sibling and for first babies are similar, the researchers said.
They add that the effects of short or long gaps on maternal and child health should motivate health professionals around the world to increase their efforts to provide family planning advice.
"The results of our systematic review could be used by reproductive clinicians around the world to advise women on the benefits of delaying a subsequent pregnancy for approximately two to five years to improve the health of both mother and the next infant."
Mr Patrick O'Brien, a spokesman for the Royal College of Obstetricians and Gynaecologists, said: "This is a huge study, providing more reliable data than any we have seen before."
He added: "Most couples will think carefully about subsequent pregnancies.
"But women and couples will take this paper into account when making their decision.
"Already, women don't want to have babies too early because of their careers or too late because of declining fertility.
"This suggests women and couples will also want to leave space between pregnancies of about 18 months."
But he said women who fell pregnant after either a very short or a long gap should not worry, as doctors could provide the right care for them to ensure the baby was healthy.
^ Back to top | BBC Sport Home | BBC Homepage | Contact us | Help | ©
~RS~q~RS~~RS~z~RS~51~RS~)