According to authors Sonia Hernandez-Diaz et al. in the study “Topiramate use early in pregnancy and the risk of oral clefts” [1]:
The increased risk of oral clefts associated with use of Topiramate early in pregnancy was more pronounced in women with epilepsy who used higher doses.
[The study’s objective was] to assess the relative risk of oral clefts associated with maternal use of high and low doses of Topiramate during the first trimester for epilepsy and nonepilepsy indications.
Science Daily reports [2]:
Approximately one out of every 1,000 infants are born with an oral cleft, but among infants exposed to low doses of Topiramate (median 100-mg daily dose) in the first trimester, that risk was 2.1 out of every 1,000 live births. Among women taking higher dose Topiramate (median 200-mg daily dose), the risk was much higher — 12.3 for every 1,000 live births.
Women with epilepsy on Topiramate have the highest relative risk of giving birth to a baby with cleft lip or cleft palate, likely due to the higher doses of Topiramate when used for controlling seizures.