A study in New Zealand found that mothers who sleep on their backs during pregnancy are prone to giving birth to babies with low birth weight, so a pregnant woman is advised to sleep on one side.
Sleeping in dahr reduces blood supply to the uterus, according to a study of 1,760 pregnant women.
Children with low birth weight are almost three times as likely to give birth.
Low birth weight can increase the risk of certain disorders, such as diabetes and cardiovascular disease, as adults.