If the whether question has been answered yes, the when question is the next planning decision, and it has real structure. The two dominant inputs are biological — female fertility declines with age, irreversibly, and the decline accelerates after 35 — and economic-developmental: parenting is easier with more resources, more stable careers, more emotional maturity, and more relationship history. These two inputs pull in opposite directions for most couples. Biology says earlier. Career, finances, and relational seasoning say later. The when conversation is the negotiation between these two pressures, and the wrong answer in either direction is genuinely costly.
The cultural script on timing has shifted dramatically in two generations. Average age at first birth in the U.S. has moved from roughly 21 in 1970 to about 27 today, and is significantly higher among college-educated women in major cities. Elizabeth Gregory's research on "later motherhood" documents that women who have children at 35+ are, on average, more financially stable, more career-established, and more relationally seasoned than younger first-time mothers — and they also face higher rates of fertility difficulty, miscarriage, and pregnancy complications. Both halves of that statement are true. Honest planning requires holding both, not picking one.
Law 4 — Plan — applied to the when question is about acknowledging that timing has consequences in multiple domains simultaneously, and that optimizing for any single domain typically degrades the others. Optimize purely for fertility certainty: have children at 25, before the career has stabilized, before the relationship has been tested, before the financial cushion exists. Optimize purely for career stability: have children at 38, when the fertility math has gotten harder and the energy reserves for sleepless nights are lower. Optimize purely for relationship seasoning: wait until you've been together a decade, which may push past the biological window. There is no answer that wins on every axis. The planning move is to choose which trade-offs you can absorb, not to pretend you can avoid trading.
The first practical input is biological data, specific to the couple. Generic fertility statistics are misleading; couples should know their actual fertility markers — AMH levels, ovarian reserve, semen analysis, any known genetic factors, any family history of early menopause or fertility difficulty. These tests are routine and cheap relative to the decision they inform. Couples who skip them are making the when decision on population averages rather than personal data, which is malpractice given how easily the personal data can be obtained.
The second practical input is career stage. Some careers have a clear "after this milestone" window — after residency, after tenure, after the founder lockup, after the partner-track decision. Others don't. For careers without natural windows, "after my career stabilizes" can mean indefinitely. The planning move is to specify the milestone: not "when my career is settled" but "after the Q3 promotion cycle" or "after the next contract renewal." Vague timing tied to vague milestones produces drift, and drift in this question is a decision in itself.
The third practical input is relationship age and quality. Couples who have children in the first year of a relationship are taking a larger relational bet than couples who have been together a decade. Both can work. The shorter-history version is higher-variance: the relationship's failure modes haven't been tested. The longer-history version has lower variance on relationship survival but may have higher fertility friction. Couples should be honest about which kind of bet they're making and not pretend their situation is the average.
The fourth practical input is the economic reality of childcare and parental leave. Early-career couples typically have less money, less leave, less flexibility, and less workplace power to negotiate accommodations. Later-career couples typically have more of all four. The difference is not trivial: the first year of parenting on weak parental leave and zero childcare budget is a different experience from the first year with strong leave and a paid caregiver. Caitlyn Collins' cross-national work shows this gap is even larger when comparing U.S. families to European peers, where the state absorbs much of the early-parenting cost. U.S. couples are absorbing it themselves, and timing decisions should reflect that the household will bear nearly all the cost.
The fifth practical input is the spacing question implicit in timing: when is the first, and how does that constrain the second (and third)? Couples often plan the first child in isolation, then discover the second is constrained by the first child's age, the parent's fertility window, and accumulated parenting fatigue. The when of the first child is usually also a decision about the when of any subsequent children, and treating it as a single isolated decision underprices the cascading constraints.
The honest version of the when conversation is: specify the milestones, get the personal fertility data, price the trade-offs explicitly, choose the trade-off you can absorb, and commit to a window — not a single date, but a window of, say, eighteen months — within which the household will move from "trying" to "child arrived." Open-ended timing is drift, and drift past the fertility window is a forced no dressed as choice.