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Formula developed to predict a woman's chances of pregnancy

15 October 2012
Appeared in BioNews 677

A mathematical model has been developed that predicts the probability that a couple will become pregnant. The model uses two key factors - the woman's age and the length of time that a couple have been trying to conceive - to determine the likelihood of becoming pregnant. Researchers at the London School of Economics and Warwick Medical School in the UK say that this model could help to determine the optimal time for a couple to consider seeking fertility treatment after failing to conceive.

The study, published in the journal PLOS One, found that the longer a couple had been trying to conceive, the less likely their chance of becoming pregnant in subsequent menstrual cycles. A couple was considered subfertile if their chance of becoming pregnant in a given menstrual cycle was less than 10 percent, or less than 50 percent over the next 12 cycles.

A woman's age was also an important factor in predicting pregnancy. Older women reached the level of subfertility after fewer menstrual cycles of trying to conceive. Women aged 25 years old were considered subfertile if they failed to conceive after 13 cycles; this dropped to ten cycles in women aged 30, and six cycles in women aged 35. The likelihood of becoming pregnant for women aged 40 years and over in the first menstrual cycle of trying to conceive was less than 10 percent.

Co-author of the study Dr Peter Sozou of the London School of Economics said: 'Comparatively few young couples have low fertility, and so for these couples the most likely reason for failing to conceive in the first few cycles is simply bad luck. There is quite a good chance of conception if they keep trying'.

'But older couples are more likely to have low fertility, so it's more likely for them than for younger couples that failure to conceive after a few months is due to low fertility', he added.

The optimal time to recommend seeking fertility treatment must balance the potential unnecessary interventions against leaving it too late, the researchers explained. The current advice is that couples should consider seeking fertility investigation and treatment after 12 cycles of trying to conceive. The researchers confirmed that this guideline is 'moderately robust', but also suggested that older couples - where the woman is aged 35 or older - might benefit from seeking treatment earlier, after just six cycles of trying to conceive.

The researchers used Bayes' Theorem, a formula that takes into account how new evidence will affect the probability of an event occurring, to investigate how the number of menstrual cycles spent trying to conceive would affect the probability of conception in the next cycle. The findings also highlighted that chance played an important role in conception for all couples at all ages.

Professor Geraldine Hartshorne at the University of Warwick and a co-author of the study said: 'Many couples are not aware that chance plays a big role in getting pregnant. People expect to get pregnant when they want to, so finding out that it isn't happening can be a shock. Approaching a doctor about such a personal matter is daunting so knowing when is the right time to start investigations would be a useful step forward'.

'We can't work out exactly when, or if, a woman will become pregnant – but this analysis can predict her chances, and give a percentage estimate of pregnancy in the next cycle', he added.

The model will need further testing before it can be used reliably in the clinical setting.

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