by Heidi Green
July 27, 2009

From an early age, my youngest child has been able to sleep through just about any ruckus. “He had 40 weeks in the womb listening to his siblings to get used to their noise,” I would tell my husband. He’d laugh at what he took to be just another one of my corny jokes. But, a new study shows that I just might be right!

Research
The current study was based on the idea of habituation—the lessening of a response to repeated exposure to the same stimulus—as an indicator of memory. Habituation is thought to demonstrate learning and to require an intact, functioning central nervous system. As Dr. Jan Nijhius explains, “habituation is a form of learning and a form of memory.”

Similar studies have been conducted since 1925. In that first study, fetal movement was observed after stimulation with a car horn. (Imagine how very shocked the babies of old must have been by such loud sounds!)

This new study, conducted by Dr. Jan G. Nijhuis and colleagues, involved nearly 100 healthy pregnant women in the Netherlands. It seems to suggest that short-term memory may be present in babies-in-utero as soon as 30 weeks gestation.

The researchers completed this study with 93 healthy Dutch Caucasian women with fetuses between the ages of 28 and 38 weeks gestation. Each of the mothers filled out a detailed questionnaire at study enrollment, including demographic details, pregnancy history, and more.

Habituation was measured using a vibroacoustic stimulator and an ultrasound scanner. Every 30 seconds, a vibrating sound one second in length was applied to the mother’s abdomen above the babies’ legs. Movements of the baby within one second of application were assessed. When the baby failed to respond to four consecutive applications, they were said to demonstrate habituation. The test continued for a maximum of 24 applications, but was stopped after habituation occurred. Each mother/baby completed the test multiple times.

Results
The researchers were most interested in the habituation rate, or the number of consecutive applications before habituation was established. They looked at their data according to subjects’ gestational age. Here’s what they found:

• Almost all of the babies displayed habituation at 30 weeks gestation.

• Babies of all gestational ages were habituated to the stimulus when tested again the same day.

• Babies of 34 weeks gestation were habituated to the stimulus when presented with it weeks later. They seemed to be able to recall the vibrating sound when they experienced it again four weeks later.

• Babies who were tested at earlier ages demonstrated habituation sooner than their gestational-age peers who had not been previously tested. Those who had been tested before seemed to have kept a memory of those experiences.

Recommendations
The results of this study don’t have any practical application to everyday prenatal care or, later, parenting. But they do provide a bit of insight into the surprising level of mental development children have—and when they get it.

These researchers suggest that such studies could prove to be helpful in screening children and identifying those with delays in central nervous system development, since treatment may be more effective when started earlier.

For now, it just causes me to smile when my napping boy sleeps through his siblings’ fireworks.