The longer a pregnancy continues after reaching full-term, the higher the risk of stillbirth, a study has found.
Of the 3000 babies stillborn every year in the UK, a third appeared healthy at 37 weeks.
Researchers from the Queen Mary University of London analysed 13 studies providing data on 15 million pregnancies and 17,830 stillbirths over 28 years.
The rate increased from 0.11 stillbirths per 1000 pregnancies at 37 weeks, to 3.18 stillbirths per 1000 pregnancies at 42 weeks.
From 40 to 41 weeks, there was a 64 per cent increase in the risk of stillbirth, while neonatal mortality was significantly higher for babies born after the 41 week mark.
Women who are 41 weeks pregnant should not be alarmed, as the risk is low - equivalent to one additional stillbirth for every 1449 pregnancies, compared to delivering at 40 weeks, the researchers said.
They said: "Any mother considering prolongation of pregnancy beyond 37 weeks should be informed of the additional small but significantly increased risks of stillbirths with advancing gestation.
"There is a need to assess the acceptability of early delivery at term to parents and healthcare providers to avoid the small risk of stillbirth."
The study also said that black women at full term were found to be 1.5 to 2 times more likely to suffer stillbirth than white women.
Lead researcher Professor Shakila Thangaratinam said: "We were surprised to see how much poorer pregnancy outcomes were for black women - they were up to twice more likely to experience stillbirth than white women.
"Healthcare professionals need to take these added risks into account when developing care plans for these women."
The study is published in PLOS Medicine journal this week.
