WASHINGTON (CBS) – The Centers For Disease Control says about 3.3 million women are at risk of exposing their fetus to alcohol if they are drinking and having sex but not using birth control.

The new CDC report says that about half of all pregnancies are unplanned, and most expectant women don’t realize they are pregnant until four to six weeks after conception.

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“This means a woman might be drinking and exposing her developing baby to alcohol without knowing it,” the CDC says.

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The agency is advising that women between 15 and 44 years old drink no alcohol at all if they are trying to get pregnant or could become pregnant.

Any level of alcohol use during pregnancy can harm a developing baby, according to the CDC.

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The CDC says doctors should advise women to stop drinking if they are sexually active but not using birth control.