NEW YORK (Reuters Health) - Pregnancies that occur after a previous pregnancy delivered by cesarean section have an increased risk of ending in stillbirth, Oxford University investigators report in the March Online Early Edition of the British Journal of Obstetrics and Gynecology.

Dr. Ron Gray and colleagues at Oxford University in the UK conducted a retrospective cohort study of the Oxford record linkage study, a database of births and deaths in the Oxfordshire and West Berkshire area between 1968 and 1989.

Among 81,707 pregnancies with complete data for analysis during the 21-year study period, a history of a previous cesarean section was documented for 5,939.

There were 290 stillbirths; 113 of which were explained and 177 which were unexplained. Deaths from congenital anomalies were excluded.

The risk of stillbirth in pregnancies with no history of cesarean section was 3.5 per 1,000 deliveries. In pregnancies with a history of cesarean section, the rate was 4.6 per 1,000 deliveries. There was a relative risk of stillbirth after a cesarean section delivery of 1.30.

The hazard ratio for explained stillbirths following a cesarean section was 1.54 and the hazard ratio for an unexplained stillbirth with a previous cesarean section was 2.08.

"The increased risk was still apparent after adjustment for potential confounding factors. Similar findings have now been reported in three other studies, suggesting this association may be a causal one," Dr. Gray told Reuters Health.

Mothers who had previous cesarean sections tended to be older, with lower parity, higher social class and a higher rate of complications during the pregnancy than those with no prior cesarean sections.

The investigators estimate that "around 4% of all stillbirths in the population may be attributable to previous delivery by cesarean section."

"We would suggest that further research on potential causal pathways is required to understand why this is happening. In the meantime, pregnant women and clinicians need to be aware of the potential increased risk, but also aware that the risk to any particular pregnancy is very small," Dr. Gray said.