We studied 68 infants whose moms smoked pot while nursing. Those infants, we found, had significant delays in motor development.

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RECENTLY I gave a lecture to a group of medical students. They appeared genuinely surprised to hear about a study finding that tetrahydrocannabinol, or THC, the psychoactive ingredient in marijuana, transfers in concentrated doses into the breast milk of nursing mothers who use the substance.

The students hadn’t heard the news, reported in April by Texas Tech, though it made national headlines and was the latest such evidence to show that.

The students also hadn’t heard that marijuana exposure via breast milk might harm the developing infant — and this I understood: Even though several studies have shown evidence that THC concentrates in breast milk, only two studies, to my knowledge, have explored the effects of THC-fortified milk on infants.

Mine was one. In 1990, as a Ph.D. student at the University of Washington, I and a colleague studied 68 infants whose moms smoked pot while nursing. Those infants, we found, had significant delays in motor development. (A much smaller study in 1985 on the same topic had been inconclusive.)

More recent research tells us that pregnant and lactating women are increasingly using marijuana with the states’ legalization of the product, and that today’s pot is far more potent than that of a generation ago.

To people in the field of early child development, like me, it’s a distressing constellation of associations and possibilities. It’s also strikingly similar to what we knew, and did not know, about fetal alcohol syndrome (FAS) 30 years ago. That subject has consumed most of my career.

In contrast with THC, alcohol’s effects on fetuses and infants have been studied intensely and widely promoted. As a result, today the Centers for Disease Control and Prevention, the U.S. surgeon general and the American College of Obstetricians and Gynecologists support the public-health message that no amount of alcohol consumption can be considered safe during pregnancy.

That’s why it is particularly disheartening to witness the recent pendulum swing against this public-health message. Research publications, blog posts and even books have communicated that “light” drinking is OK for pregnant women. (“Light” is undefined, but generally it’s viewed as one drink a day.)

Research, including my study of twins, challenges this conscience-easing trend.

In reviewing 25 years of data from our FAS diagnostic clinic, it’s clear to me that one drink per day can place a fetus at significant risk. One of every 14 children diagnosed with FAS at our clinic had a reported exposure of one drink per day.

We have no way of knowing which fetuses are more vulnerable because risk is not based solely on a mother’s alcohol consumption. My study of twins, which is completed but not yet published, strongly suggests that genetics plays a pivotal role.

When genetically identical twins are exposed to alcohol, they are identically impacted. If one has FAS, so does the other. But when genetically nonidentical, or fraternal, twins are exposed to the same levels of alcohol, one twin can be born unaffected while the other can experience growth deficiency, structural and functional brain damage, and unique facial features.

Other researchers are in the early stage of exploring this, too. But the surgeon general’s advisory to abstain from alcohol during pregnancy aligns with genetic rationale to protect all fetuses, including the 1 in 14 who, for reasons we don’t fully understand, is especially vulnerable to alcohol’s impact.

In the end, whether a mom-to-be considers a puff of pot or a sip of wine, I have to ask one question: Given what we know, and more importantly, what we don’t, why take the risk?