Counseling alone is not enough to help most pregnant women quit smoking, a review of eight studies concludes.

The analysis included studies that enrolled a total of 3,300 pregnant women. The studies evaluated whether counseling helped the women quit after six months. Four of the studies showed no difference between pregnant women who received smoking-cessation counseling and those who did not. The other four studies found women who didn’t receive counseling were only slightly less likely to quit than those who received counseling, according to Reuters.

The study with the highest success rate found 24 percent of women who received counseling quit smoking, compared with 21 percent who didn’t receive counseling. The researchers recommend further study on the safety and effectiveness of nicotine replacement therapy for pregnant women, to see if these aids will boost their smoking cessation rates.

According to the Centers for Disease Control and Prevention, a national survey found approximately 13 percent of U.S. women reported smoking during the last three months of pregnancy. The CDC notes that babies born to women who smoke during pregnancy are about 30 percent more likely to be born prematurely than babies born to nonsmokers. They are also more likely to be born with low birth weight.

The analysis appears in the British Journal of Obstetrics and Gynaecology.