Formula developed to predict a woman's chances of pregnancy

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.