A woman's fertility starts declining as early as her late 20s - not in her 30s as was previously thought, according to a study published in Human Reproduction*, Europe's leading journal of reproductive medicine. The finding comes from a US-Italian study of 782 healthy couples who were using natural family planning methods to avoid pregnancy.
US-based lead author Dr David Dunson said that, to his knowledge, this was the first study to observe that a woman's fertility started to decline before the age of 30. However, he reassured women that this did not mean a lower overall probability of achieving pregnancy if they delayed trying to become pregnant until their late 20s or early 30s, but it meant that it may take a month or two longer to become pregnant than it would have in their early 20s.
"Although we noted a decline in female fertility in the late 20s, what we found was a decrease in the probability of becoming pregnant per menstrual cycle, not in the probability of eventually achieving a pregnancy," he said.
The study found evidence that it is not only women who need to be concerned about their biological clock; men's fertility also begins to decline from as early as their late 30s.
However, there was some encouraging news for older would-be parents - age did not decrease the duration of the fertile window - the 6 days within the menstrual cycle when women were most likely to become pregnant.
The study, carried out by the National Institute of Environmental Health Sciences in North Carolina and the University of Padua in Italy, aimed to evaluate the effects of male and female age on natural fertility by controlling for variation in sexual behaviour. According to the authors, most analyses of age-related changes in fertility cannot separate effects due to reduced frequency of intercourse from effects directly related to ageing. But this study was designed to be able to do that by controlling for frequency and timing of intercourse**. This makes our results uniquely reliable," said Dr Dunson.
From their data the researchers estimated the chances of a woman becoming pregnant following intercourse on a given day relative to ovulation. Assuming intercourse occurred at the peak time for conception (2 days prior to ovulation) and presuming that their partners were the same age, women aged 19-26 had around a 50% chance of pregnancy in any one menstrual cycle. This fell to around 40% for women aged 27 to 34. For women aged 35-39 it was less than 30% if the woman had a partner the same age, but fell further to around 20% if the man was five years older (because his fertility had by then also started to decline).
"Nearly all pregnancies fell within the fertile window and, on average, the day-specific probability of pregnancy declined for women from the late 20s onwards being around twice as high for women aged 19 to 26 as for women aged 35 to 39. When we controlled for the age of women, we found that fertility was significantly reduced for men aged over 35," Dr Dunson said.
He added that it was not surprising to find a decline in female fertility this early as many researchers felt that decline with age during reproductive years was continuous, though gradual.
"Despite this overall trend of declining fertility we found a lot of variation among healthy couples in their fertility. There was enormous heterogeneity that is not accounted for by age. Even focusing on the 50% of couples having closest to average fertility, the range in the probability of pregnancy on the peak day of the fertile window extends from 20% to 60%. Epidemiological studies have identified some of the factors associated with variability, including prenatal exposures, sexually transmitted disease history, smoking and occupational exposures, but much of this heterogeneity remains unexplained."