STUDY QUESTION Until what age can couples wait to start a family without compromising their chances of realizing the desired number of children? SUMMARY ANSWER The latest female age at which a couple should start trying to become pregnant strongly depends on the importance attached to achieving a desired family size and on whether or not IVF is an acceptable option in case no natural pregnancy occurs. WHAT IS KNOWN ALREADY It is well established that the treatment-independent and treatment-dependent chances of pregnancy decline with female age. However, research on the effect of age has focused on the chance of a first pregnancy and not on realizing more than one child. STUDY DESIGN, SIZE, DURATION An established computer simulation model of fertility, updated with recent IVF success rates, was used to simulate a cohort of 10 000 couples in order to assess the chances of realizing a one-, two- or three-child family, for different female ages at which the couple starts trying to conceive. PARTICIPANTS/MATERIALS, SETTING, METHODS The model uses treatment-independent pregnancy chances and pregnancy chances after IVF/ICSI. In order to focus the discussion, we single out three levels of importance that couples could attach to realizing a desired family size: (i) Very important (equated with aiming for at least a 90% success chance). (ii) Important but not at all costs (equated with a 75% success chance) (iii) Good to have children, but a life without children is also fine (equated with a 50% success chance). MAIN RESULTS AND THE ROLE OF CHANCE In order to have a chance of at least 90% to realize a one-child family, couples should start trying to conceive when the female partner is 35 years of age or younger, in case IVF is an acceptable option. For two children, the latest starting age is 31 years, and for three children 28 years. Without IVF, couples should start no later than age 32 years for a one-child family, at 27 years for a two-child family, and at 23 years for three children. When couples accept 75% or lower chances of family completion, they can start 4-11 years later. The results appeared to be robust for plausible changes in model assumptions. LIMITATIONS, REASONS FOR CAUTION Our conclusions would have been more persuasive if derived directly from large-scale prospective studies. An evidence-based simulation study (as we did) is the next best option. We recommend that the simulations should be updated every 5-10 years with new evidence because, owing to improvements in IVF technology, the assumptions on IVF success chances in particular run the risk of becoming outdated. WIDER IMPLICATIONS OF THE FINDINGS Information on the chance of family completion at different starting ages is important for prospective parents in planning their family, for preconception counselling, for inclusion in educational courses in human biology, and for increasing public awareness on human reproductive possibilities and limitations.

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Human Reproduction
Department of Public Health

Habbema, D., Eijkemans, R., Leridon, H., & te Velde, E. (2015). Realizing a desired family size: When should couples start?. Human Reproduction, 30(9), 2215–2221. doi:10.1093/humrep/dev148