Mothers who take recreational drugs during their pregnancy are more likely to have a baby with birth defects, according to a recent study.

After testing 517 mothers in English inner city hospitals, researchers from the University College London found that nearly 15 percent had taken recreational drugs during pregnancy and that mothers of babies with birth defects of the brain were significantly more likely to have taken drugs than mothers with normal babies.

"Our findings suggest a link between brain birth defects and recreational drug use in expectant mothers," Anna David, lead author of the study, said in a statement. "We were unable to identify significant links between specific drugs and brain birth defects. Therefore I would discourage women trying to get pregnant and those in early pregnancy from taking any recreational drugs including cannabis. Since only 20 of the mothers in our study had babies with brain birth defects, a larger study of such cases is now needed to examine the links with specific drug use more closely."

For the study, researchers took hair samples from consenting mothers, which were then tested for evidence of recreational drug use. The study included 213 women whose baby had a type of birth defect with potential links to recreational drug use, 143 women whose baby had a birth defect with no previously reported links to drug use and 161 women whose baby was normally formed.

Evidence of drug use was found in a significantly higher proportion of women, 35 percent, whose babies were born with brain birth defects, compared to women whose babies were normally formed, 13 percent. Brain birth defects included brain anomalies other than spina bifida, such as brain cysts and under-development of the brain. These can have severe consequences and lead to lifelong conditions such as cerebral palsy.

About 15 percent of the women who agreed to take part tested positive for at least one type of recreational drug. Drug use was highest around conception and reduced as the pregnancy progressed, but around half of the women who smoked cannabis continued to do so throughout the second trimester.

Smaller studies had suggested that drug use might be a primary risk factor for gastroschisis, a defect in the baby's belly that must be surgically repaired at birth. Other known risk factors for this abnormality include young maternal age and smoking. This larger study showed that the young age of the mother rather than recreational drug use was identified as the primary risk factor for gastroschisis. But for brain defects, drug use was a primary risk factor after taking into consideration the mother's age and use of tobacco and alcohol.

Researchers said larger studies are now needed to investigate the link between the types of drug use and brain birth defects.

"Current evidence linking recreational drug use with birth defects is patchy as it relies on self-reporting which can be unreliable," David said. "Our anonymized hair testing offers an objective measure of recreational drug use and showed that it is common in pregnancy."

The findings are detailed in the journal PLOS One.