Doctors in Scotland followed nearly 31,000 women who went to the hospital for a miscarriage in their first pregnancy and subsequently became pregnant between 1981 and 2000.
Among women who got pregnant within six months, 85 percent had a healthy baby. Among women who waited more than two years to get pregnant again, the rate was 73 percent. The study was published Friday in the medical journal BMJ and was partly funded by the Chief Scientist's Office in Scotland, an agency of the Scottish government.
"It's unnecessary for women to wait to conceive again after a miscarriage," said Sophinee Bhattacharya, a lecturer in obstetric epidemiology at the University of Aberdeen in Scotland, one of the paper's authors.
She said current guidelines from the World Health Organization, which recommend women delay getting pregnant for at least six months after a miscarriage, should be changed. Bhattacharya said WHO guidelines are based on a study from Latin America, where women usually have children at an earlier age than in the West.
Because women in developed countries often wait until they are older to have children, Bhattacharya said any delays to conception could reduce the chances of a healthy baby.
Women over 35 are more likely to have problems getting pregnant, and women over 40 have a 30 percent risk of a miscarriage. Normally the miscarriage rate is about 20 percent.
Bhattacharya said it was possible the guidelines on how long to wait after a miscarriage could vary for different countries.
In an accompanying commentary, Julia Shelley, an associate professor of health and social development at Deakin University in Australia, said research from more sophisticated hospital records are needed before changing advice. She wrote that birth registries, like the one used in the study, are often incomplete and cannot answer the question of how long women should wait after having a miscarriage to conceive again.
Britain's Royal College of Obstetricians and Gynaecologists doesn't advise women to wait a certain amount of time before trying for a baby after a miscarriage.
"If you wish to be pregnant, trying again soon, whenever you feel physically and emotionally ready, does not increase your risk of miscarrying the next time," said Dr. Tony Falconer, the college's president-elect.