New research reveals a surprising connection between heavy alcohol use and unintended pregnancy. This is true even among women who strongly wish to avoid getting pregnant. The study found that women who drank heavily were 50% more likely to become pregnant within one year. This is in comparison to women who drank moderately or not at all.
In contrast, cannabis use did not appear to increase the risk of unintended pregnancy, even among frequent users.
A Closer Look at the Numbers: What the Study Found
Researchers studied over 2,000 non-pregnant women aged 15 to 34, focusing specifically on 936 participants who expressed a high desire to avoid pregnancy.
Of those 936 women:
- 429 reported heavy drinking, based on standard alcohol screening
- 362 reported cannabis use, including 157 who used it daily or almost daily
Over the course of a year, 71 women became pregnant, despite wanting to avoid it. More than half of those pregnancies occurred among heavy drinkers.
Heavy Drinkers Were More Likely to Become Pregnant
Out of the 71 pregnancies:
- 38 were among heavy drinkers, compared to just 33 among moderate or non-drinkers
- That’s more than half of all unintended pregnancies in the study group
This suggests that heavy alcohol use may undermine contraception behavior, decision-making, or other pregnancy prevention strategies, even when motivation to avoid pregnancy is high.
Cannabis Use Not Linked to Increased Pregnancy Risk
Interestingly, the study found no significant link between cannabis use and unintended pregnancy.
Of the 71 pregnancies:
- Only 28 occurred among cannabis users
- Cannabis users had similar pregnancy rates as those who did not use cannabis at all
This finding challenges earlier assumptions that all substance use carries equal reproductive health risks.
Women Who Drank Heavily Were Also More Motivated to Avoid Pregnancy
Another unexpected finding: heavy drinkers and frequent cannabis users were more motivated to avoid pregnancy than those who used substances less often.
This suggests a disconnect between intention and outcome, especially among women who drink heavily.
Lead author Dr. Sarah Raifman from UCSF School of Medicine explains, “Women who drink heavily really want to avoid pregnancy, but still face higher risk.”
Why Might Heavy Drinking Lead to More Unintended Pregnancies?
Researchers are still investigating why heavy drinking increases the risk of pregnancy, but several theories exist:
- Alcohol impairs judgment, possibly reducing effective contraceptive use
- Drinking may lead to unprotected sex, especially in social or party settings
- Forgetfulness or misusing birth control may be more common during periods of intoxication
Dr. Raifman adds, “Finding out why those pregnancies happen is the next step in our research.”
Health Risks of Alcohol Use During Early Pregnancy
One reason this research matters is because early unrecognized pregnancies combined with alcohol exposure can have serious consequences.
Fetal alcohol spectrum disorders (FASD) occur when a fetus is exposed to alcohol, especially in early development. These disorders can affect brain function, behavior, and growth.
Because the risk of FASD increases with the amount and duration of drinking, unintended pregnancies among heavy drinkers carry added health concerns.
Clinical Implications: What Doctors Should Know
Given these findings, the authors recommend that clinicians talk with patients who drink heavily about pregnancy prevention and health risks.
Dr. Raifman advises, “It’s important for doctors to support women who drink heavily to stop drinking as soon as they suspect pregnancy.”
Routine screening and open, judgment-free conversations about substance use and reproductive goals could help reduce unplanned pregnancies and fetal exposure to alcohol.
The Bigger Picture: Unintended Pregnancy Remains a Public Health Challenge
This study highlights a larger issue: despite increased access to contraception, unintended pregnancy remains common, especially among younger women.
When alcohol or substance use complicates contraception behaviors, the risk grows—even when women are motivated to avoid pregnancy.
Public health efforts must account for behavioral and situational factors, not just access to birth control.
Heavy Drinking Is a Hidden Risk Factor in Unintended Pregnancy
This new research adds to a growing body of evidence that substance use—especially heavy alcohol consumption—may interfere with reproductive choices.
Women who drink heavily are not only at greater risk of becoming pregnant unintentionally, but also may unknowingly expose fetuses to alcohol before they realize they are pregnant.
With cannabis use showing no similar pattern, the study opens up new questions for researchers, healthcare providers, and policymakers alike.
Key Takeaways: What the Study Teaches Us
- Heavy drinking increases unintended pregnancy risk by 50%, even among women who actively want to avoid pregnancy
- Cannabis use does not appear to increase pregnancy risk in the same way
- Women who use alcohol or cannabis may still strongly desire to avoid pregnancy
- Health providers should screen for substance use when discussing contraception and reproductive goals
- More research is needed on why these pregnancies occur and how to prevent them
