The study from University of East Anglia (UEA) also found that women who felt they were being supported by a partner were less likely to start smoking again.
"More women quit smoking during pregnancy than at any other time -- but as many as 90 percent start again within a year of their baby being born. This is particularly true among women in lower socio-economic groups," explained lead researcher Dr Caitlin Notley from UEA's Norwich Medical School.
Researchers studied interviews with more than 1,000 new mothers and found that the stress of caring for a newborn, sleepless nights, social pressure and the idea that they no longer need to protect the baby -- all contribute to relapse.
"We found that many women see smoking as a way a coping with stress. They also believe that physiological changes influence cigarette cravings, and that they no longer need to protect the baby from smoking's harmful effects," Dr Notley noted.
The study sought to find out what factors influence women to start smoking again after childbirth, as well as what things make it easier or more difficult to stay smoke free.
Feeling low, lonely, tired, and coping with things like persistent crying were also the triggers.
"Women reported that cravings for nicotine, which had lessened or stopped during pregnancy, returned," she added.
Social influences also play a part.
"Because social interaction is especially valued after childbirth, some women reported that their friends expected a return to smoking which influenced them to start again," the authors noted.
Supportive partners are cited as key to remaining smoke free, along with personal praise from health professionals.
The paper appeared in the journal Addiction.
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