People Born Very Prematurely Have 50% Lower Chance of Being Parents, Study Says

Huge study finds same problem in prematurely born girls and boys, though whether the obstacle is medical or social problems, or both, is not clear

A premature baby at Tel Aviv's Ichilov Hospital.
David Bachar

People born very prematurely have a significantly reduced probability of having children themselves, a vast new study done in Israel has shown.

Most studies on premature birth until now checked morbidity and brain function in later life. The new study looked at the generation beyond, based in part on data from the Health Ministry.

Being born weighing less than 1,500 grams (3.3 pounds) reduced the chances of becoming a parent in adulthood by 50 percent compared with people born within the normal weight range, concluded the study, which was done at Jerusalem’s Shaare Zedek Medical Center in collaboration with the Health Ministry.

The study was based on hospital records of more than 67,000 babies born between 1982 and 1997. Premature birth is defined as occurring before the 37th week of pregnancy, subdivided into more mature babies (weeks 34 to 37) weighing 1,500 grams to 2,500 grams, and less mature ones, born before the 34th week and weighing less than 1,500 grams.

Preemie survival rates have improved very much, especially of the smallest infants born at less than 1,500 grams, said Prof. Sorina Grisaru-Granovsky of Shaare Zedek, who led the study: "The question was what happens when they have children.”

The researchers used records of all live births from 1982 (adults now 35 years old) to 1997 (adults now 20 years old). They looked at 377 low-weight babies born weighing less than 1,500 grams, 3,737 midrange babies botn (1,500 grams to 2,500 grams) and 54,000 normal-weight babies (2,500 grams to 3,800 grams).

Survival into adulthood was lowest for the lowest-weight babies (61 percent), compared to babies in the middle range (97 percent). And when they wanted children: “We found that the group with the lowest weights had a 50 percent lower chance of reproducing compared to adults who had been born with a normal weight,” says Grisaru-Granovsky.

This applied equally to men and women. Results for the midrange weight group were not significantly different from the normal weight group.

The study has its limitations: it only tested subjects at birth and when they became (or didn’t become) parents. No medical or socioeconomic data for these subjects was checked. Furthermore, the later years included in the study deal with 20-year-olds, which may be too young to assess for reproductive success.

However, the results point to long-term implications of very low birth weight.

Why prematurely born people have such reduced chances of parenthood was not checked. It could be the result of medical problems, including damage to the reproductive system, and/or social implications of the low-weight premature birth, surmise the doctors.

Very small preemies that survive may have difficulties as adults including cerebral palsy or blindness. Some can't live independently, Grisaru-Granovsky points out. "Premature birth can also affect the pituitary-gonad axis in ovaries or testicles, which can lead to reproductive insufficiency. This is seen mainly in girls, but also in boys. These babies also undergo various surgeries due to problems such as perforated intestines, and boys born before the 32th week of pregnancy need surgical repair of undescended testicles, which may affect their reproductive capabilities."

Also: people born prematurely with a midrange weight are 2.4 times more likely to give birth prematurely themselves. This may be because people in this group may have been born prematurely due to poor placental perfusion (blood dispersal), a problem which may repeat itself in the next generation.

Currently, 0.9% of live births in Israel are very premature, with babies weighing less than 1,500 grams.