Drinking during pregnancy may not be as simple as was once thought, a new Yale study has found.

While the surgeon general has proscribed all prenatal alcohol consumption since 1988 for causing birth defects and developmental problems, the recent study showed that low-to-moderate prenatal alcohol consumption in the first and third trimesters of pregnancy is not associated with lower birth weights, smaller birth lengths and head circumference, congenital anomalies or preterm delivery — some of the physical symptoms associated with Fetal Alcohol Syndrome. Their findings suggest that there is a threshold at which alcohol causes fetal damage, challenging the previously held notion that any alcohol consumption could lead to malformed embryos. The findings were published in the Annals of Epidemiology on Oct. 15.

“In order for alcohol to act as a teratogen [an agent that causes malformation of the fetus], the alcohol has to build up a high enough concentration in your blood stream to pass through the placenta to reach the baby,” said study co-author and professor of obstetrics, gynecology and reproductive sciences at the Yale School of Medicine Jessica Illuzzi. “But there’s no one amount of alcohol that causes this — it depends on the women’s metabolism.”

Since there is no single amount of alcohol that is acceptable, she admitted that it is a risky message to send patients.

While many studies have examined the adverse effects that heavy drinking has on fetal development, this study sought to examine the spectrum of alcohol consumption. The study also looked at various elements of nutrition during pregnancy including caffeine, alcohol and certain micronutrients. It evaluated 4,496 women, recording what they ate and drank during pregnancy to avoid any recall bias. The data show that there was no link between low-to-moderate prenatal alcohol consumption and pre-term delivery, infant head size, length and weight.

In fact, there was actually a curvilinear relationship with the lowest levels of consumption, meaning that the best birth outcomes came with low levels of alcohol consumption — not any alcohol consumption at all.

But Katherine Campbell, who has a fellowship in maternal fetal medicine at the medical school, said these findings should be interpreted with caution.

“I don’t know if anyone would say at this point that [low amounts of alcohol] are really protective against [the dependent variables measured in the study] because when you do an epidemiological study, it’s possible that the population that they’re studying already has protective effects,” Campbell said.

In other words, although the study corrected for certain variables like education, race and age, more variables could be at play behind the improved birth outcomes.

Due to budget constraints, the study could not collect extensive data from the second trimester, so Illuzzi said she would like to see future studies examine all three trimesters. She would also like to see studies that follow the babies through childhood to see if they exhibit symptoms of FAS, which cannot be diagnosed until later in a child’s life.

“It’s important for future research to investigate the mechanisms of prenatal alcohol exposure across the spectrum of low level exposure to chronic exposure” said Lisbet Lundsberg, a research scientist in obstetrics, gynecology and reproductive sciences at the medical school and a co-author of the study. That would include drinking patterns and beverage types.

Despite the findings, Campbell said she will not alter her message to patients to avoid alcohol altogether during pregnancy. She said that the data are important because they can be comforting to women who drank before realizing that they were pregnant and want to know their risks. But since it is unclear how much alcohol is safe, it is better to refrain from drinking completely, she said.

According to the Fetal Alcohol Syndrome Website, FAS is the leading non-genetic cause of mental and behavioral problems in children.

Correction: Dec. 8

A previous version of this story misstated the position of the surgeon general on prenatal alcohol consumption.