Cephalexin is a Pregnancy category B medication. In the case of cephalexin, that means reproduction studies have been performed on mice and rats using oral doses of cephalexin 1.5 times the maximum daily human dose and have revealed no harm. There are, however, no adequate and well-controlled studies in pregnant women. Basically that means they can't rule out possible fetal harm, but it's very unlikely.
I suspect if your OB has prescribed cephalexin for you, he/she has probably used it before with expectant mothers and feels it is safe. If you're still worried, though, don't hesitate to call your doctor and express your concerns.
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