Giving birth to a heavyweight baby can more than double a woman's risk of breast cancer, a study has found.
The association may be caused by hormonal changes to the womb environment, scientists believe.
Researchers in the US looked at data on 410 women taking part in the Framingham Offspring Birth History Study. Around 7.6% of the women were later diagnosed with breast cancer. Those who gave birth to the heaviest infants were two and a half times more at risk of developing the disease.
Another study which collected data on almost 24,000 pregnancies showed hormonal changes likely to affect infant birth weight and breast cancer risk. The findings are reported in the online journal Public Library of Science ONE.
Lead researcher Professor Radek Bukowski, from the University of Texas, said: "We found that women delivering large babies - those in the top quintile (fifth) of this study, which included babies whose weight was 8.25 or more pounds - have increased levels of hormones that create a 'pro-carcinogenic environment'.
"This means that they have high levels of oestrogen, low levels of anti-oestrogen and the presence of free insulin-like growth factors associated with breast cancer development and progression. Women can't alter their pregnancy hormones, but can take steps to increase their general protection against breast cancer."
Smoking cannabis can double the risk of a woman giving birth prematurely, according to another study in the same journal.
Researchers in Australia and New Zealand looked at premature delivery risk factors in more than 3,000 pregnant women. They found that marijuana use prior to getting pregnant more than doubled the likelihood of a premature birth.
A strong family history of low birth weight increased the risk almost six-fold.
"Our study has found that the risk factors for both forms of pre-term birth vary greatly, with a wide variety of health conditions and histories impacting on pre-term birth," said study leader Professor Gus Dekker, from the University of Adelaide.