Short first pregnancies and having a heavier firstborn child may slightly increase a mother’s risk of breast cancer, a new study has found.

Overall, in the long-term, pregnancy reduces the risk of the disease for most women—but having children can affect a woman’s risk in different ways.

In a new analysis of more than 80,000 women who had given birth, scientists found that a short pregnancy was associated with an increased risk of breast cancer, as was having a heavier firstborn child.

Scientists at The Institute of Cancer Research, London, analyzed detailed information on pregnancies of 83,451 women who took part in the Breast Cancer Now Generations Study.

They investigated the effect of length of pregnancy and birth weight of a woman’s first child on breast cancer risk, taking into account a wide range of other variables linked to breast cancer, such as alcohol, smoking, and breast-feeding.

Of the women enrolled in the study, 2%—1,767—went on to develop breast cancer; but, those whose firstborn child was heavier, or whose pregnancy was shorter, had a higher risk of the disease.

Short pregnancy linked to increased risk of breast cancer

Women whose firstborns were heavier than 4,500 g (9 lb, 14.7 oz) were 53% more likely to go on to develop breast cancer than those whose firstborns weighed between 3,000 g (6 lb, 9.8 oz) and 3,500 g (7 lb, 11.4 oz).

A shorter pregnancy was also linked with an increased risk of breast cancer with those who had given birth between 26-32 weeks of gestation, being 30% more likely to develop the disease in their lifetime than those who had given birth at 40-41 weeks.

The effect was stronger for premenopausal breast cancer, with women who had given birth between 26-32 weeks more than twice as likely to develop breast cancer before menopause than those who had given birth at 40-41 weeks.

The study was published in the journal Breast Cancer Research last week, and funded by Breast Cancer Now.

While the study found a statistically significant increase in breast cancer risk in women with heavier firstborns and shorter pregnancies, the exact size of the increase could still fall within a fairly wide range and needs to be further established.

'Further studies are needed'

The authors believe that a pregnancy that ends at an earlier stage could lead to increased risk through fast cell growth and high sex hormone levels in the first and second trimesters of pregnancy.

They suggest that higher levels of estrogen and a protein called IGF1 during pregnancy in women with heaver firstborns could help explain the increased risk of breast cancer in these women, although this possible link will need to be further investigated.

Professor Anthony Swerdlow, professor of epidemiology at The Institute of Cancer Research, London, said:

“Many lifestyle, genetic, and other factors together contribute to a woman’s risk of breast cancer, and having children is known to have a complex effect on breast cancer risk.”

“In our analysis of a large cohort of women who had given birth, we found that both a shorter pregnancy and a higher birth weight in a woman’s firstborn child raised her risk of developing breast cancer.”

“Further studies are needed to estimate more precisely the extent to which a shorter pregnancy and heavier firstborns increase women’s risk of developing breast cancer, and to study the possible causal mechanism in more depth.”