The complete resource for NICU families from admission to discharge and beyond

Nicu Baby

It seems like a simple question – one that everyone asks! The books you read – even the clothes you buy – use your baby’s age as a reference point. But, when you have a premature baby, like everything else, it’s not that easy. For the next several years, your baby will have 3 ages, each one meaning something different and each one important. Here’s a description…

Chronologic Age – based on birthdate:

This is the age that most people use to answer the question “How old are you?” This age refers to how long it’s been since you were born. To get to be “old,” you must have lived for those days. So, for example, on June 1st, a baby born on May 1st is now one month old. You have to live a whole year before you get to be one year old. No matter how premature you are, your chronologic age is how long it’s been since you were born. Health care professionals use chronologic age to administer immunizations.

In the NICU, we count in days of life (abbreviated DOL). You may see this, or hear the care team refer to it. The day you are born is Day Of Life (DOL) #1. The next calendar day is DOL #2 (even if you were born just before midnite!) – and you are now one day old. “Days old” is always one less than DOL, although it really doesn’t matter except to be confusing!

Post-conceptional (or post-menstrual) age -- based on due date:

This is the age that is used the most in premature babies, and it relates to the degree of prematurity at birth. A full-term pregnancy is 40 weeks from the last menstrual period (or 280 days), and development from a tiny embryo into a baby ready to live outside the womb progresses at a predictable rate. When that baby is born early, development only up to that point has occurred. Thus, we don’t expect a baby born at 25 weeks’ gestation (EGA – estimated gestational age) to be as developed as a baby born at term. Our best hope is that the preterm infant’s development will occur at the same rate it would have occurred had the pregnancy continued. Sometimes it goes slower but it rarely goes faster. The NICU staff determines the care of your baby on his/her post-conceptional age, so that our expectations are reasonable. The more premature your baby, the greater the gap between chronologic age and post-menstrual age (PMA).

For example, the first “developmental milestone” of a term baby is called the “social smile,” – and it occurs about 2 months of (chronologic) age. That’s 2 months for a term baby – or 48 weeks PMA (40 weeks at birth + 8 weeks of life). For the baby born at 25 weeks EGA, at 8 weeks of age, s/he is only 33 weeks PMA – still not even close to due date! Thus, it’s unreasonable to expect that baby to show a social smile. If development proceeds at the expected rate, the 25 week infant should show a social smile after 23 weeks of life – or almost 6 months old! (Don’t sweat the math – your NICU team can help you with it.) Use your due date to help you – that’s the date your baby is 40 weeks PMA. Once you’ve passed that date, you can use it to reference developmental milestones listed in the babybooks.

Developmental Age:

The third age is your baby’s developmental age, or what baby milestones s/he is exhibiting. This age probably won’t become relevant until after you take your baby home. We hope that NICU babies develop at the same rate as if they were born at term, but this doesn’t always happen. Extreme prematurity and/or other complications can delay a baby’s development. As mentioned above, the first of these “develomental milestones” is a social smile, followed by rolling over, crawling, cruising, and other motor skills. At the same time, during the first years of life, your baby is developing language and cognitive skills – babbling, saying his/her first words, learning the word “no.” We know the ages at which each of these skills normally appears, and your pediatrician will assess your baby’s progress at every visit. Your baby’s developmental age refers to the milestones s/he is demonstrating and the age at which those milestones typically appear. When a baby is developing normally, his/her developmental age is the same as his/her post-menstrual age. Each human being is different, and some variation is typical. However, too wide a gap between developmental and post-menstrual ages can be an early sign of brain abnormalities. Be sure to keep your appointments with the high-risk follow-up clinic, government early intervention program, pediatrician or developmental pediatrician you have been referred to. The earlier a delay is spotted, the better chance of minimizing or eliminating it!

A couple of other notes about age:

  • If you conceived with assistance, you may know the precise date of conception. To maintain consistency among the medical community, your maternal-fetal specialist adjusted your due date to be consistent with a post-menstrual, rather than a post-conceptional age. Thus, on the day of conception, you were already 2 weeks pregnant!!!
  • Over time, “ages” tend to converge so that your child will be acting (developmentally) his/her chronologic age. The textbooks say this happens by 2 years of life, but many children take longer to “catch up.” For many, they catch up developmentally but remain physically small for age. Let them stay that way – Catch-up growth can jeopardize their long-term health as adults. (See fetal hypothesis for further discussion.)

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