Women who delivered live births in 1996 responded to a mailed questionnaire approximately 2 to 6 months after delivery (N =17,378). Data from 10 US states were included in the study, and the overall participation rate was 75%. Overall, 25.6% of women reported cigarette smoking before pregnancy. Among women who smoked before pregnancy, 44.5% quit during pregnancy. Among women who quit during pregnancy, half relapsed by the time of the survey.
Carmichael SL, Ahluwalia IB. Correlates of postpartum smoking relapse. Results from the Pregnancy Risk Assessment Monitoring System (PRAMS). Am J Prev Med. 2000 Oct;19(3):193-6. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&lis
t_uids=11020597&dopt=Abstract

Women are much more likely to quit smoking around the time of pregnancy than at any other. While the prevalence of smoking has also decreased among pregnant women, 12% of pregnant women report current smoking. Unfortunately, women who quit during pregnancy have extremely high rates of relapse during the months immediately following delivery Haas JS et al. Bupropion in breast milk: an exposure assessment for potential treatment to prevent post-partum tobacco use. Tobacco Control 2004;13:52-56 http://tc.bmjjournals.com/cgi/content/full/13/1/52?ijkey=7394c22fca9eb4c
a640beb5719f1e12495e7a2c2#R5

An estimated 39 percent of White women who had smoked before pregnancy quit smoking while pregnant. Of the women who quit, 70 percent resumed smoking within one year of delivery. Of those who relapsed, 67 percent resumed smoking within three months of delivery and 93 percent within six months. Fingerhut LA et al. Smoking before, during, and after pregnancy. American Journal of Public Health, Vol 80, Issue 5 541-544 http://www.ajph.org/cgi/content/abstract/80/5/541?ijkey=e43f5a3e3fc4a11a
7b3ab7ca2762296863c594a8&keytype2=tf_ipsecsha

Of women who quit smoking beginning before the 20th week of pregnancy and continued abstinence through delivery (n = 134), only 37 percent were still non-smoking at 6 months postpartum. Mullen PD et al. Maintenance of nonsmoking postpartum by women who stopped smoking during pregnancy. American Journal of Public Health, Vol 80, Issue 8 992-994
http://www.ajph.org/cgi/content/abstract/80/8/992?ijkey=5d50132b92069b9f
a68ea0263bb9ab62c6f496a9&keytype2=tf_ipsecsha

While it has been established that many women quit smoking during pregnancy, little is known about postpartum maintenance of smoking cessation and relapse. A sample of 567 women were surveyed at about 6 months postpartum in order to (a) describe the patterns of smoking relapse; (b) assess the perceived importance of smoking cessation in pregnancy and postpartum; and (c) identify potentially high risk situations for smoking relapse. Results indicate that about half (56%) of the women who quit smoking during pregnancy relapse to smoking by 30 days postpartum. McBride CM, Pirie PL. Postpartum smoking relapse. Addict Behav. 1990;15(2):165-8. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&lis
t_uids=2343790&dopt=Abstract

A substantial proportion of women who quit smoking during pregnancy will relapse in postpartum. 106 women who quit smoking during pregnancy were surveyed by telephone after the 28th week of pregnancy, and at 6 weeks and 6 months postpartum. Point prevalence estimates of smoking relapse indicated that at 6 weeks and 6 months follow-up, 24% (n = 26) and 40% (n = 42), respectively, of the women had returned to regular smoking. McBride CM, Pirie PL, Curry SJ. Postpartum relapse to smoking: a prospective study. Health Educ Res. 1992 Sep;7(3):381-90. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&lis
t_uids=10148743&dopt=Abstract