From birth, all the eggs a woman will ever get are stored in her ovaries. Her stock of eggs diminishes throughout life – some mature during ovulation, while others die as a kind of collateral damage, spurring the process along. Her cache dwindles until too few eggs remain to maintain fertility. This point is menopause and it is inevitable.
Three elements – the number of eggs in stock, the rate at which eggs mature (or die) and the threshold of eggs needed for ovulation – affect the age of menopause.
The timing of menopause is partly affected by genetics. On top of this, the vagaries of early life – such as poor nutrition, infection, lack of education, even living in a rural place or at high altitude – can lead to an earlier menopause. The number of children a woman gives birth to or whether she smokes can also tweak the timing.
Early menopause is linked to a risk of osteoporosis and cardiovascular diseases, while later menopause carries a risk of breast cancer and womb cancer.
Out in the real world, we kind of assume menopause hits ‘around 50 ish’ and that’s that. In a study published this week in the American Journal of Human Biology, researchers looked at the systematic differences in menopause timing for Spanish women in Madrid compared to the city’s Latin-American immigrants.
Lead researcher Irene Pérez-Alcalá and colleagues tested whether migration to another country can change the timing of menopause.
In Madrid, the team found that Spanish women went through menopause at around 52 years of age, but the average (or, for stats geeks, the median) age of the Critical Period for Latin-American immigrants was 50.5 years – one and a half years earlier.
Within the Latin-American group, women from Ecuador went through menopause the latest, at a median age of 51.4 years. The earliest menopause, on average, was experienced by Bolivian women at the age of 48 years. Women from Peru, Colombia, Dominican Republic and other countries fell in between this range.
One of the most interesting results (for my limited and vaguely male brain, at least), was that women who menopaused in their birth countries were more likely to have menopaused (I am determined to make this a verb) earlier compared to women who migrated before menopause.
This study surveyed 274 Spanish-born women and 301 Latin-American immigrants between 45 and 55 years of age. The number of women answering the surveys was small – only 22 Bolivian women, up to 72 women from Ecuador. So one shouldn’t draw grand, sweeping conclusions, but the results suggest something interesting is going on among the migrants.
The researchers touch on two possible explanations for why migrating women go through a later menopause than those who migrated after menopause. The first being that mostly the healthiest (and so, latest to menopause) migrate.
The other explanation goes by the tongue-twistingly wonderful name of the Latino Epidemiological Paradox. The Paradox is an observation in the US that immigrants live as-healthy or healthier lifestyles than US-born white people, despite – on average – lower incomes and less education.
As with many biological processes, menopause is a complicated twirl of genetics, development and life lived among other people. Even its existence is an evolutionary mystery, because if an organism can no longer reproduce, what is its use, evolutionarily-speaking?
This study is cool because it shows just how nuanced, just how personal the menopause is, given the battering or the beatification taken from a lifetime.
Reference: Pérez-Alcalá, Sievert, Obermeyer and Reher – 2013. Cross cultural analysis of factors associated with age at natural menopause among Latin-American immigrants to Madrid and their Spanish neighbors. American Journal of Human Biology http://dx.doi.org/10.1002/ajhb.22447