Pregnancy and child

New fertility formula 'predicts pregnancy' chances

The Daily Mail reports that a ‘new formula predicts woman’s chance of falling pregnant’.

The story is based on research into a common problem faced by many couples – is the fact that they have failed to conceive a baby just ‘bad luck’ and they will eventually have a baby, or is it the sign of a deeper, underlying problem? Unfortunately for worried couples, neither the news stories, nor this research, provide any new answers.

The researchers developed a complex mathematical formula in order to estimate a couple’s chances of falling pregnant in a given month (menstrual cycle) that was based on two main factors:

  • the number of cycles that they had been trying to conceive
  • the woman’s age – as women grow older it becomes more difficult to conceive a baby

They found that what is often considered subfertility – trying to conceive without success for one year – is a fairly reasonable indication of fertility problems.

Based on the model’s predictions for women over the age of 35, the researchers consider that it may be more appropriate to start thinking about investigation and possible treatments earlier for those over the age of 35, than for younger women – possibly after six months.

It is difficult to draw conclusions from the current study. The mathematical formula is highly complex and its reliability will need to be assessed with further testing. The main finding will not come as any great surprise, either to fertility experts, or the general public: female fertility declines with age.

Where did the story come from?

The study was carried out by researchers from Warwick Medical School and other institutions in the UK and Germany. Individual researchers received research expenses from a Value in People Award from the Wellcome Trust, and RWTH Aachen University.

The study was published in the open-access peer-reviewed scientific journal, PLOS ONE.

The media coverage is representative of this study and the formula it has created, but fails to take the overview of the situation:

  • that this is a not a formula that could be used by individual couples
  • it is not known whether the computer model will be brought into any clinical use, but it is unlikely in the foreseeable future without further study
  • the study doesn’t change current advice for people experiencing trouble conceiving 

What kind of research was this?

The researchers observed that previous studies have found that the longer it takes a couple to conceive, the more their chances of conceiving in subsequent months go down.

However, the ideal scenario would be to:

  • identify those who are likely to experience greater difficulties so they could be treated early
  • delay unnecessary (and sometimes expensive) investigations and treatment in couples who are more likely to eventually conceive without assistance

This was a mathematic modelling study where the researchers have attempted to design a formula that could predict a couple’s chance of getting pregnant within any given menstrual cycle. The model also takes into account how long the couple have been trying to conceive and the woman’s age. 

This study is of scientific and medical interest, but currently has no direct implications for clinical practice. More studies would be needed to test how well it works in practise, and whether couples’ conception outcomes improve if it is used as part of their care.

What did the research involve?

The researchers have designed a computer program that allows them to calculate conception probabilities, taking into account the number of cycles a couple have been trying to conceive, and the woman’s age.

The complex methods firstly discuss how to calculate the ‘intrinsic conception rate’ for any given couple – their probability of achieving a pregnancy in the next cycle, if they have not yet achieved a pregnancy. However, it is not possible to measure this precisely because the researchers say ‘current methods for fertility assessment give only imperfect information about a couple’s fertility’ and so this is an estimate only, for any given couple. Their models assume that this ‘intrinsic conception rate’ is a constant value and give examples of how the value of this figure will influence how likely the couple is to conceive. For example, 80% of couples with an intrinsic conception rate of 13.6% will conceive within 12 cycles, but 20% of couples with an intrinsic conception rate of 1.84% will conceive within the same period.

The researchers then go on to discuss how they have used statistical methods to create a ‘probability distribution’ to take into account uncertainties around a particular couple’s intrinsic conception rate.

They tested their model using four scenarios that model populations of people with different proportions of people with higher or lower fertility.

What were the basic results?

The researchers found that the number of cycles of prior attempted conception without success affects the intrinsic conception rate.

In simple terms, this suggests that the longer you have been trying to conceive without success, the less likely you are to achieve pregnancy in the next cycle. 

In their population examples, unsurprisingly, the probability of achieving pregnancy on the next cycle is greater if the population contains only low numbers of couples with low fertility. As women age, their fertility declines, so age ultimately influences how many cycles will need to elapse before the woman becomes pregnant. Their models show that for women aged from 25 to 30 there is little difference in their chances of conceiving on the next cycle, but the chances drop from the age of 35 upwards.

How did the researchers interpret the results?

The researchers conclude that ‘couples vary in their fertility’. They say that a reasonably good indicator of subfertility is around 12 reproductive cycles without conception (i.e. one year of trying to get pregnant naturally without success), but a larger or smaller number of cycles may be more appropriate depending on the population from which the couple is drawn. Ultimately, in this model, this relates to the age of the couple. The researchers say that it may be appropriate to have a lower threshold for starting fertility investigation and treatment in women above the age of 35, which they recommend at being around six months.

This advice is not actually anything significantly different from the current consensus of opinion, which already advises women over the age of 35, to seek advice sooner.


This research is of scientific and medical interest, but has no immediate implications for couples trying to conceive. The researchers say, ‘The approach has potential applications in decision support systems for determining the best course of treatment and supports the optimal use of resources’. However, it is not possible to comment on the reliability of this computer model and whether it could improve couples’ conception outcomes without further testing. Results from such studies would be needed before we know whether this program would be a useful addition to fertility care. Also,  the complex mathematical formula used in this paper is not something that individual couples would be able to use themselves.

Overall, the main finding of this study is not going to come as any great surprise either to fertility experts, or the general public: fertility declines with age.

NHS Attribution