The first review of its kind published on Monday showed a direct connection to smoking while pregnant and birth defects, like clefts, clubfoot and other deformations, although one in five pregnant Americans continue to smoke, TIME said.

Researchers at University College London proved that smoking during pregnancy can cause deformations, such as cleft lip and cleft palate, as early as when an embryo transforms in the third week of pregnancy. Pregnant non-smokers exposed to secondhand smoke are also at risk.

The study published in the journal Human Reproduction Update from the European Society of Human Reproduction and Embryology examined nearly 174,000 malformed infants, finding that chemicals in cigarettes including nicotine, tar and carbon monoxide directly influence whether or not a child will be born with a defect.

Not only can smoking during pregnancy lead to skull, limb, feet, gastrointestinal and optical deformities, but the study points to smoking affecting the rate of miscarriages and premature births as well as the ability for smokers to get pregnant initially. Smokers looking to get pregnant are at high risk for ectopic pregnancies, vaginal bleeding and more, according to a report in TIME.

The authors of the publication close with a statement urging women to quit smoking before or, at worst, during pregnancy.