Do you have kids? When are you having kids? How old are your kids?

In One and Only: The Freedom of Having an Only Child, and the Joy of Being One,” author Lauren Sandler points out how our questions about other people’s children are often asked as plurals. “Kids” not “kid” is the default assumption.

“If a kid has no siblings,” Sandler writes, “it’s assumed that there’s a hush-hush reason for it: that parents don’t like parenthood (because they are selfish), or they care about their status – work, money, materialism – more than their kid (because they are selfish), or they waited too long (because they are selfish).

A new study in the journal Pediatrics suggests that one of those reasons is that the child’s birth was traumatizing for parents. Whether or not a child is “first” or “only” depends in part on how early he was born.

Researchers at Finland’s National Institute for Health and Welfare (THL) identified all 230,308 recorded singleton infants born in Finland between January 1987 and September 1990 and interviewed those infants’ parents.

The study revealed that parents of infants born preterm were less likely to have subsequent children than parents whose born at term. Infants born “extremely” preterm (between 23 and 27 weeks) were the least likely to have a subsequent sibling, but even those born nearly at term (between 34 and 36 weeks) were less likely to have a subsequent sibling. The researchers concluded that for every 1,000 preterm births, there were 142 “missing siblings” from parents who would have been statistically likely to have more children.

A THL press release put the results in simple terms: “The more premature a child is born, the greater the probability that it will be the last child in the family.”

The researchers have not determined a cause for this lowered birth rate among parents of preterm infants, but speculate that the lowered rate “may reflect the crisis a premature birth may cause for the parents and its far-reaching impact. The birth of a premature infant is often a surprise, and can place the parents in a situation where their hopes and resources do not meet their expectations on parenting or the challenges during early childhood.”

In other words, parents’ experiences with their preterm children – which may include harrowing weeks or months in neonatal intensive care units as well as lifelong health problems – may make them more hesitant to have more children. The sole exception were parents whose children born preterm died within their first year. Those parents were actually more likely to have subsequent children.

In a December post to the Pediatrics’ blog, editor-in-chief Lewis First stresses that the issue will require further study before researchers can draw a causal link between preterm birth and the subsequent birth rate.

In the interim, however, we might want to consider the pain inflicted by probing questions about subsequent children. There’s no need to ask a family member when she’s planning to have more kids. Asking a stranger at the grocery store “Is he your first?” suggests, however innocently, that a parent ought to have a “second.” Instead of asking these kinds of close-ended questions about family planning and family size, we could all do better by asking open-ended questions about the kid who is actually right there in front of us.