Snoring during pregnancy was also linked to higher rates of Caesarean delivery.
Experts said snoring may be a sign of breathing problems that could deprive an unborn baby of oxygen.
Previous research has already shown that women who start to snore during pregnancy are at risk from high blood pressure and the potentially dangerous pregnancy condition pre-eclampsia.
More than a third of the 1,673 pregnant women recruited for the new US study reported habitual snoring.
Scientists found that women who snored in their sleep three or more nights per week had a higher risk of poor delivery outcomes, including smaller babies and Caesarean births.
Chronic snorers, who snored both before and during pregnancy, were two thirds more likely to have a baby whose weight was in the bottom 10%.
They were also more than twice as likely to need an elective Caesarean delivery, or C-section, compared with non-snorers.
Dr Louise O’Brien, from the University of Michigan’s Sleep Disorders Centre, said: “There has been great interest in the implications of snoring during pregnancy and how it affects maternal health but there is little data on how it may impact the health of the baby.
“We’ve found that chronic snoring is associated with both smaller babies and C-sections, even after we accounted for other risk factors. This suggests that we have a window of opportunity to screen pregnant women for breathing problems during sleep that may put them at risk of poor delivery outcomes.”
Women who snored both before and during pregnancy were more likely to have smaller babies and elective C-sections, the researches found. Those who started snoring only during pregnancy had a higher risk of both elective and emergency Caesareans, but not of smaller babies.
Snoring is a key sign of obstructive sleep apnoea, which results in the airway becoming partially blocked, said the researchers, whose findings appear in the journal Sleep.
This can reduce blood oxygen levels during the night and is associated with serious health problems, including high blood pressure and heart attacks.
Sleep apnoea can be treated with CPAP (continuous positive airway pressure), which involves wearing a machine during sleep to keep the airways open.
Dr O’Brien added: “If we can identify risks during pregnancy that can be treated, such as obstructive sleep apnoea, we can reduce the incidence of small babies, C-sections and possibly NICU (neo-natal intensive care unit) admission that not only improve long-term health benefits for newborns but also help keep costs down.”
