Fetuses in the womb scowled after their mothers ate kale but smiled after they ate carrots, according to a new study of around 100 pregnant women and their fetuses in England.
The study offers a rare look at how fetuses respond to flavors in real time.
The researchers gave the participating women capsules containing powdered versions of the two foods. Thirty-five women consumed the equivalent of one medium carrot, and 34 women consumed the equivalent of 100 grams of chopped kale. The remaining 30 women didn't consume either.
Twenty minutes later, ultrasound scans showed that most of the fetuses exposed to the kale flavor seemed to grimace, while most exposed to the carrot appeared to be laughing. The control group, meanwhile, didn't have the same responses.
"We are the first ones who could actually show on an ultrasound scan the facial expressions in relation to the food which the mother has just consumed," said Nadja Reissland, a co-author of the study and the head of the Fetal and Neonatal Research Lab at Durham University.
Previous research has shown that the amniotic fluid surrounding a fetus can have different smells or flavors depending on a pregnant person’s diet.
A 2001 study also found that infants who were exposed to the flavor of carrots through amniotic fluid or breast milk showed fewer negative facial expressions in reaction to carrot-flavored cereal than infants who hadn't had those previous exposures. But the study examined the infants' responses only outside the womb.
The fetuses in the new research were at 32 to 36 weeks’ gestation. (An average pregnancy lasts 40 weeks from the last menstrual period.)
The ultrasound images suggest reactions similar to those of kids or adults who taste something bitter, Reissland said, but it's not known whether fetuses actually experience emotions or dislikes in a similar manner.
The grimaces in the ultrasounds "might be just the muscle movements which are reacting to a bitter flavor," Reissland said.
She added, however, that fetuses are known to make facial expressions.
"If you look at it from 24 to 36 weeks’ gestation, their expressions become more and more complex," Reissland said.
Dr. Daniel Robinson, an associate professor of neonatology at Northwestern University Feinberg School of Medicine, who wasn’t involved in the new research, similarly cautioned that people shouldn’t interpret the ultrasound images as showing the fetuses' happiness or distaste.
He noted, however, that "there is the notion that newborns and infants will have a preference for sweeter flavors, and there are evolutionary ideas behind that."
In designing the new study, Reissland and her team chose powdered kale and carrots over juices or raw vegetables for a few reasons. For one, powder made it easier to ensure that each participant consumed the same number of calories. (The women were also asked not to consume anything containing carrots or kale on the day of the scan.)
Reissland said the capsules were also used because some pregnant women couldn't stand the flavor of kale, and the researchers were worried that their negative reactions would influence the fetuses' responses.
"I had a number of people in the lab, and I tried to give them a kale juice to drink, and you should have seen the expressions," Reissland said.
Third, the capsules helped prevent the flavor from getting too diluted while it was being processed in the body.
"The bitter flavor gets into the small intestines and then into the mother’s blood and then into the placenta and the amniotic fluid," Reissland said. "This process seems to take around 20 minutes, and what you then get is a specific reaction of the fetuses to that flavor."
Reissland thinks her study could improve our understanding of how exposure to flavors in the womb affects eating habits later in life. If a fetus repeatedly tastes kale in the womb, for instance, that baby may be more likely to tolerate — or even enjoy — the flavor once it starts eating solid food.
Robinson said scientists have already found that exposure to different types of foods in the first few months of life "can help with willingness or acceptance of foods later in infancy."
"Diet during pregnancy is really important and influential on the health of not just the developing fetus, but the future for that child," he said.
Mothers who have healthy diets while they are pregnant may also find that their babies are less fussy eaters, Reissland said.
"If we can actually get [children] to like green vegetables and to perhaps not like sweets that much, it might help with regard to their weight gain and their weight balance," she said.