Studies have shown that pregnant women derive plenty of health benefits from engaging in regular moderate to vigorous exercise. But there has been some concern about whether mom’s fitness might be coming at the expense of baby’s. A new study now suggests that babies also show fitness gains, albeit it more subtly.

Exercise physiologist Linda May of Kansas City University of Medicine and Biosciences in Missouri and her colleagues recruited 26 women to have the fetal-lung movements of their babies recorded periodically during the last trimester of pregnancy. Half of the women engaged in regular cardiovascular exercise at least three times a week for 30 minutes or more at a time. The others women were “not couch potatoes,” May says, but didn’t go out for organized, heart-pumping exercise.

Once every four weeks during the latter months of their pregnancy, these women would come into the lab. There, the researchers would use magnetocardiography for 20 minutes each visit to record movements in the womb which correspond to a baby’s heart beats, sucking (yes, babies innately practice sucking), limb movements and contractions of the fetal diaphragm, which causes an expansion of a baby’s lungs.

The last type of movement, which occurs periodically and for progressively longer periods as a baby approaches birth, amounts to practiced breathing. It’s practice because the baby gets all of the oxygen it needs from mom’s blood. But an infant’s lungs have to be prepared to take their first breath seconds after birth. And they won't be successful if they haven’t regularly been going through the motions in prep for their natal day.

Because mom’s exercise tends to diminish her resting heart rate, some physiologists had worried that this might diminish the amount of oxygen getting to baby — perhaps mildly starving the fetus of oxygen. And that’s what May’s group set out to investigate.

The monitoring technology they used assays magnetic fields that correspond to electrical fields given off by the baby’s body. Those electrical fields vary with movement. They also get progressively harder to pick up as the baby gets bigger. By contrast, May explains, the magnetic fields associated with the electrical fields actually strengthen as baby grows, offering a more precise window into fetal movements.

The developing brain and nervous system spontaneously trigger the diaphragm exercises. Earlier studies by others had characterized what these movements tend to look like, both in terms of frequency of events and the pattern of diaphragm movements during an event.

For instance, some 20 weeks into pregnancy, a baby may begin some erratic attempts, each lasting only a few seconds, to expand the lungs. By 36 weeks, the events can last minutes, beginning with some fluttery, shallow “breaths,” which eventually develop into deeper more regular ones. Then, as the exercise ends, the breathing movements turn shallow and more irregular before eventually dying out.

At the Experimental Biology meeting in New Orleans, today, May reported that babies whose moms had been engaging in regular vigorous workouts were different: They exhibited lung movements resembling those of babies closer to birth. In fact, she said, their breathing movements “are characteristic of a more mature nervous system.”

This finding is the exact opposite of what would be expected if the babies weren’t getting adequate oxygen, she says.

The bottom line: Babies aren’t hurt by their moms’ regularly exercising, May concludes; if anything, they appear to benefit.

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