Consuming small amounts of alcohol normally doesn't carry a dire warning, but the Centers for Disease Control and Prevention says millions of women could be putting their developing babies at risk because of their drinking habits.

An estimated 3.3 million women who drink are sexually active but not on birth control, according to a CDC report released Tuesday. And three out of four women who want to get pregnant don't stop drinking alcohol when they stop using birth control.

The CDC warns that consuming alcohol during pregnancy can lead to fetal alcohol spectrum disorders, which encompass a range of behavioral, intellectual and physical disabilities. There is no known amount of alcohol that's safe to consume while pregnant, according to the agency.

"Alcohol can permanently harm a developing baby before a woman knows she is pregnant," CDC Principal Deputy Director Anne Schuchat said in a statement. "About half of all pregnancies in the United States are unplanned, and even if planned, most women won't know they are pregnant for the first month or so, when they might still be drinking. The risk is real. Why take the chance?"

That's advice echoed by the American Academy of Pediatrics, which issued guidance last year saying that no amount of alcohol is safe to consume at any point during a pregnancy. The organization noted that the odds of a child developing FASD increase by 12 times when a mother drinks during the first trimester, compared to not drinking at all.

AAP pointed to surveys showing that 8 percent of Americans drink while pregnant and another study showing "increased risk of infant growth retardation even when a pregnant woman's consumption was limited to 1 alcoholicdrink per day."

A pregnant woman who drinks alcohol passes it along to her baby, which doesn't develop a liver until later stages of the pregnancy.

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This kind of "why risk it?" warning to pregnant women has been criticized by some who say that telling women to completely abstain from drinking shames those who have an occasional drink. There are also disagreements over evidence showing the effects of very moderate drinking later in a pregnancy.

But the CDC warns that the risks are too high, especially given that most women don't know they are pregnant until they are four to six weeks along. The agency says women should stop drinking when they stop using birth control.

"Every woman who is pregnant or trying to get pregnant - and her partner - want a healthy baby," Coleen Boyle, director of CDC's National Center on Birth Defects and Developmental Disabilities, said in a statement. "But they may not be aware that drinking any alcohol at any stage of pregnancy can cause a range of disabilities for their child. It is critical for healthcare providers to assess a woman's drinking habits during routine medical visits; advise her not to drink at all if she is pregnant, trying to get pregnant or sexually active and not using birth control; and recommend services if she needs help to stop drinking."