Harsh environments early in development are associated with lower fertility later in life. Such an understanding now seems to stand vindicated in a UK study covering Bangladeshi women who migrated at different stages of life.
Five groups of women were compared, including those who grew up in Bangladesh but moved to the UK as adults, those who had moved as children and others who were born in either England or Bangladesh and remained there.
Women who had spent their childhood in the healthier conditions of the UK had higher levels of reproductive hormones than those brought up in Bangladesh.
The researchers found statistically significant differences in progesterone levels between individuals who had migrated as children, second-generation migrants, and women of European descent, as compared to the levels for Bangladeshi women who had never migrated and women who migrated as adults. Progesterone levels for the first three groups were higher than those for the Bangladeshi women who had not migrated or who had migrated as adults. The age at which women migrated also seemed to be linked to their progesterone levels.
Amongst women who migrated before the start of menstruation, those who migrated at a younger age had higher average progesterone levels. However, this relationship did not seem to hold true for women who had migrated after they started menstruating.
The research team said in the UK women would have access to better sanitation and health care, and less risk of disease.
While higher hormone levels could increase a woman's ability to conceive, it could also have some negative impact.
The significant increase in progesterone levels might also in higher breast cancer risks in subsequent generations of this community. The findings suggest that early childhood is critical in determining the rate at which girls mature and how high their reproductive hormone levels are as adults.
Lead researcher Dr Alejandra N˙˝ez de la Mora said girls who migrated early in life seemed to mature more quickly. This could be because they were living in an environment where they had access to a better diet and general health. He said the female body could monitor its environment throughout childhood to gauge when and at what rate it would be best to mature.
At the same time researchers caution that it is not certain whether the lower average progesterone levels of the particular groups studied would actually translate into lower fertility. It is possible that although certain individuals and groups had lower levels of this hormone, they may in fact have been as fertile as individuals with higher levels of the hormone.