Erasmus MC researchers, participating in an international study, have developed a mathematical model that can assist couples in having the number of children that they would ideally like. The model predicts the age at which couples should start having children depending on whether they want one, two or three kids. The results have just been published in the journal Human Reproduction [1].

The model takes into account how important it is to the couple to have the number of children that they would ideally like. If they would prefer to have two children, for example, and if they hope for a 90 percent probability of success, then they should start when the prospective mother is about 27 years old. If they are planning to use IVF if necessary, then they can wait until she is 31 years old. If the couple is satisfied with a lower probability of success of 75 percent (and again hoping for two children), then they should start trying to conceive when the prospective mother is 34 years old, or 35 if they are intending to use IVF.
The latest starting age is strongly dependent on the number of children the couple wishes to have. Couples who wish to be 90 percent sure of having three children should start trying to conceive at 23 years of age, whereas couples who want only one child can wait until the prospective mother is 32. If a couple plans to use IVF if necessary, then they can tack on four additional years, just as in the case of couple hoping for two children.
The model was developed to assist couples in planning the size of their families. These days, most couples wait much longer before having children than they did in the past, and oral contraceptives mean that family planning can be done very precisely. Today’s young people want to complete their studies, build a career for as long as possible before having kids, and engage in all kinds of enjoyable activities (such as travel) that are simply far more complicated once children come into the picture. Couples tend to postpone having kids for these kinds of reasons, although they are then left guessing whether they will be able to have the kind of family they have in mind. “From the point of view of sound decision-making, couples need to have insight into the probability of having the number of children that they would ideally like to have. The results of our research can help couples to make these kinds of decisions,” explains lead researcher Dik Habbema of the Department of Public Health at Erasmus MC.
This is the first time that such a mathematical model has been developed for family planning purposes. The model takes into account the desired number of children, the couple’s conviction regarding their ideal family size, and whether the couple will opt for fertility treatments. The article in Human Reproduction contains a large number of calculations taken from the study. The examples in this press release have been drawn from these results. 

An Advance Access version of the full article is published online here.

[1] “Realizing a desired family size: when should couples start?”, by J. Dik F. Habbema et al. Human Reproduction, doi:10.1093/humrep/dev148