New research has found that women who use marijuana during their pregnancies can cause more harm to their babies than many smokers originally thought.

The findings, published in JAMA Pediatrics, are based on 51 studies of 21 million women. It showed that using marijuana during pregnancy is linked to a 52% higher risk of pre-term delivery and 75% higher risk of low birth weight, which is considered 5 ½ pounds or less at delivery.

It can also increase a woman's chance of miscarriage or the chance of their baby dying during or right after birth.

While organizations such as the March of Dimes has released information on the warnings against smoking marijuana, data has revealed that the number of pregnant people who use marijuana has increased in recent years.

An analysis of more than 450,000 pregnant women in the U.S. found that marijuana use more than doubled between 2002 and 2017.

Experts say a majority of women surveyed said they felt marijuana it was natural and safe. Doctors say that is not true, as it contains many of the same toxins, irritants and carcinogens as cigarette smoke.