Premature Babies and Corrected Age

Michael and Ian are both 12 years old. Their birthdays are only a day apart. They are in the same class in school and play on the same soccer team. Both are reluctant to show any interest in girls or romance. Neither has begun a rapid growth spurt and both are still sopranos. Their parents and teachers think of them as a lot alike.

But… let’s go back about 12 years. Michael was born on just the day he was predicted for, about 40 weeks from the first day of his mother’s last menstrual period before she became pregnant (depending on the mother’s cycle, this puts the birth about 38 weeks after conception). Ian, however, was born about 35 weeks after the first day of his mother’s last menstrual period  (about  33 weeks after conception). Michael was a full term (or just “term” baby), while Ian was what most people call “premature”, a preterm baby. They were quite a lot different at that time, because they were at different gestational ages—40 weeks for Michael and 35 weeks for Ian..

The most obvious difference between them was their length and weight at the time of birth. At 7 pounds 15 ounces, Michael was similar to many babies at 40  weeks of gestational age. He was average in weight for his gestational age. Ian was also average for his gestational age of 35 weeks, but because he had not been developing as long, that average weight was not as great as  Michael’s. The average weight at 35 weeks gestational age is only about 5 and ½ pounds, and Ian weighed 5 pounds 7 ounces. Michael was at the average age for birth, counting from his conception, and he was similar in weight to most babies born at that gestational age. Ian, even though he was much lighter in weight than Michael, was about the same as most babies born at the same gestational age as himself.

There were some other differences between the newborn Michael and the newborn Ian. One was that Ian had a good deal more of the creamy skin coating called vernix; Michael had had time to lose this and just showed a bit in skin creases. Ian still had a lot of the head hair called lanugo, but Michael had only a little. When Ian was lying on his back, an examiner could take the baby’s left hand and pull it to the right quite a way across his chest and neck. Michael, lying in the same position, would resist the pull and the arm could not be pulled very far. Again, when they were lying on their backs, Ian’s foot could be brought up close to his ear, but Michael’s could not. (There were many other differences, and you can see more about them at www.ballardscore.com.)

But—very confusingly—Michael and Ian were about the same chronological age! The same amount of time had passed since they were born. Ian spent a little extra time in the hospital, but even a while after he came home in good condition, his father, Sam, was worried. “I looked in the book,” Sam said. “It says right here that a baby who is two months old can look at people and even smiles at them sometimes. Our friends’ baby Michael does that all the time, but Ian is still just looking around a little bit, just like Michael did a month ago. I don’t know what’s going on. I know they say not to compare babies, but I just don’t see how two babies almost the same age can be so different unless something’s wrong somewhere.”

What Sam missed was that even though they were born only a day apart, the babies Michael and Ian were not really the same age. Two months after their birth, Michael had been developing for the 38 weeks between his conception and his birth, plus two months—about 46 weeks. Ian had been developing for the 35 weeks between his conception and his birth, plus two months—about 41 weeks.  This does not seem like a big difference, but it means that at two months Michael was almost 10% older than Ian, developmentally speaking. (But of course as time passed the percentage difference became less and less, and by the time the boys were 12 the percentage difference was a very tiny one.)

It’s much less confusing to understand preterm babies if when they are very young we think of them by their corrected age—the gestational age plus the chronological age. If Sam had done that, he would have realized that when he compared Ian’s behavior to Michael’s from a month before, he was exactly right because Ian was developmentally at about the point where Michael had been a month earlier.

There are risks for preterm babies, and their development can be affected by their too-early birth. Also, whatever the reason for the early birth, that reason could also contain risks for the babies. It would be a mistake to think that all differences between all preterm and term babies just have to do with corrections for gestational age. But, all other things being equal, the biggest differences will be present simply because of gestational age differences, and understanding that  fact can save a lot of worry for parents of young babies.