Most couples have some kind of timeline in mind for when it feels right to have a baby. Maybe it’s vague, maybe you haven’t talked it through as a couple to land on a precise target, but you likely have a sense of what you think needs to happen first and what conditions you think would be optimal for a good start. “We’re going to pay off some debt and explore Colorado a little more and then get started,” we used to tell people. “We’re thinking we’ll try in a couple of years depending on how work is going,” or “We’re going to squeeze in another degree before we have kids,” we’ve heard others say.

Conventional wisdom says timing is everything—it’s essential to find the optimal time to launch your family. “Now that the baby is only a theoretical possibility rather than a biological inevitability, the pre-requisites for baby-readiness in the mind of the modern couple grow every year,” wrote Read and Rachel Schuchardt. Fifty years ago, nearly three-quarters of couples had children within three years of getting married. Now, only about a third do so.

It seems that more and more couples believe that if they get going too soon they’ll get themselves and their babies off to a bad start. Admittedly, there are few things in life more daunting than launching a new life into the world. Anyone who soberly reflects on the magnitude of the venture and of the things that could go wrong can be motivated to think more cautiously about their timing. But for today’s couples, the factors guiding timing have grown more complex.

Couples have always worried about being able to provide for a new family—economic changes, job situations, and debt issues have always been considerations. Today couples are more likely to go into marriage with much greater consumer and educational debt than their parents did, leading many to put off having children. In fact, the percentage of college graduates citing education debt as their reason for delaying children nearly doubled between 1991 and 2002. Additionally, many now have the mentality that getting established—a common prerequisite for having children—means attaining the standard of living that their parents spent decades accumulating.

The promise of a longer life also complicates a couple’s timeline. People who only expected to live for sixty to seventy years knew their life span would affect the amount of time they would be able to spend with their offspring. In the midst of what Robert Butler calls a “longevity revolution,” however, it’s a lot easier to think about starting a family at a much later age.

Adding greater complexity to a couple’s timeline is the growing perception that reproductive technology can make it possible for a woman to become a mom just about whenever she wants. Where the limits of fertility once seemed unyielding, they now seem highly flexible.

In the face of ballooning debt, ever promising breakthroughs in artificial reproductive technologies, and faith that we can live longer than our forebears, couples have more reasons than ever to delay starting their families, alongside few if any cautions about how long they wait. In such conditions, a more stretched out timeline seems prudent and ideal for both them and their future baby. But is it?

Our concern is that even couples with the best intentions tend to underestimate the power of inertia, while overestimating the flexibility they actually have in their timing.  …

[This is an excerpt from the “When” section of Start Your Family: Inspiration for Having Babies.]