Want Two or Three Kids? When to Start Trying, Working Backward From Family Size

The math behind family size is simple once you see it. Each child takes time: months or more to conceive, nine months of pregnancy, and a recovery and spacing period before trying again. If you want three children, you will be trying for your last one several years after your first, and fertility at that later age is what sets the limit. That is why people who want larger families often need to start earlier than people who want one child.

What the Habbema Model Says

In 2015, J. Dik F. Habbema and colleagues published a model in Human Reproduction that simulated couples trying for one, two or three children. It estimated the latest age at which the female partner could start trying and still have a given chance of reaching the family size she wants, with and without IVF as a backup. Here are the model's published results:

Ages are the female partner's latest starting age. Source: Habbema et al., Human Reproduction, 2015 (Table I).

How to read the table

• Pick your row by how much certainty you want. The 90% rows suit couples for whom family size is very important. Couples comfortable with more uncertainty can look at the 75% or 50% rows.
• IVF buys a few years, not unlimited time. Across the table, having IVF as a backup moves the starting age later by roughly one to five years.
• These are averages. The model is based on population data. Your own chances depend on your health, your partner's fertility, and things no model can see.
• It is about the female partner's age. Male age matters too, though more gradually; the model focuses on the woman's age because it has the larger effect.

What the model does not cover

Every model simplifies. This one estimates chances for couples trying to conceive with the female partner's own eggs. It does not tell you about:

• Known fertility problems. If either partner has a diagnosis such as PCOS, endometriosis, blocked tubes or a low sperm count, your timeline may look different, in either direction once treated.
• Donor eggs. With donor eggs, the egg donor's age matters more than the recipient's for the chance of pregnancy.
• Single parents and same-sex couples using donor sperm, whose path to each pregnancy may involve insemination or IVF from the start.
• Your health in pregnancy. Later pregnancies carry somewhat different health considerations, which your doctor can discuss.

If any of these apply, use the table as background and ask a fertility doctor to help you build a personal plan.

Working Backward: A Simple Planning Exercise

If you and your partner have not yet talked through how many children you want, our 25 questions to discuss before trying start with exactly that.

Spacing and Age Gaps

Spacing affects both the timeline and health. ACOG and the Society for Maternal-Fetal Medicine advise avoiding an interval of less than six months between a birth and the next conception, and discussing the risks and benefits of intervals shorter than 18 months. Our explainer on how long to wait between pregnancies covers the reasoning.

That guidance is a floor for health, not a target. Many families choose a gap based on work, childcare, finances and what they want their children's relationship to be like. But if you want three children and are starting in your 30s, tight spacing may be part of the trade-off, so talk it through with your doctor.

When the First Child Takes a While

If it takes longer than expected to have your first child, it compresses the timeline for later ones. ASRM recommends a fertility evaluation after 12 months of trying if the female partner is under 35, and after 6 months if she is 35 or older. If you are planning a larger family, it is reasonable to ask for an evaluation sooner rather than later.

Also, getting pregnant easily the first time does not guarantee the same the next time. Secondary infertility is real; see our secondary infertility guide.

Options That Can Extend Your Timeline

• Egg freezing. Freezing eggs at a younger age can give a later attempt a better starting point, though it is not a guarantee. See our egg freezing guide.
• Banking embryos. Couples doing IVF sometimes freeze extra embryos for future siblings. The embryos keep the age at which the eggs were retrieved.
• Getting tested early. A baseline workup, including ovarian reserve and a semen analysis, can help you decide whether to start sooner or consider preservation.

For trying after 35, the TTC after 35 hub collects our related guides.

Tools for When You Start

Once you begin, the fertility window calculator estimates your most fertile days each cycle, and the due date calculator can help you sketch how a given conception month maps onto your timeline.

For a closer look at spacing, see LifeFertile's planning the age gap between children. ConceiveGuide's age and fertility: the real numbers and FertileStart's how long does it take to get pregnant add detail on the age curve and time to conception.

Frequently Asked Questions

The Bottom Line

If you want more than one child, plan from your target family size, not just from your age. The Habbema model suggests earlier starting ages for larger families, a few extra years if IVF is an option, and more flexibility if you accept a lower chance. Use it as a planning tool, add spacing and early evaluation to your timeline, and talk to a doctor if the numbers suggest you are short on time.