The taste of kale made unborn babies grimace, but they smiled at carrots, a new imaging study found

• A new study showed evidence of fetal reactions to different foods consumed by their mothers.
• One group swallowed kale capsules, while others ate carrots in pill form.
• Researchers used 4D imaging to see fetuses smile and scowl in response to those flavors in the womb.

Babies may be able to taste and smell before they are born, according to findings of the first study to investigate how fetuses react to various flavors in the womb.

In a study of about 100 pregnant women in the UK — published online in the journal Psychological Science — researchers observed fetuses smiling after their mothers ate carrots and scowling at kale consumption. The fetuses were between 32 and 36 weeks gestation.

This suggests that during the last three months of pregnancy, fetuses are mature enough to distinguish between different tastes of foods consumed by the mother, lead researcher Beyza Ustun told CNN.

While the findings cannot prove that fetuses prefer carrots to leafy greens, the study adds to growing evidence that suggests babies begin developing their senses of taste and smell in utero, particularly in the third trimester.

A follow-up study of the same babies post-birth is already underway with the aim of determining whether prenatal exposure affects taste preferences later on, according to a press release from Durham University.

"We think this repeated exposure to flavours before birth could help to establish food preferences post-birth, which could be important when thinking about messaging around healthy eating and the potential for avoiding 'food-fussiness' when weaning," said Utsun, a postgraduate researcher in the Fetal and Neonatal Research Lab at Durham.

A mother's diet may affect a baby's flavor preferences

Developing fetuses likely experience flavor by inhaling and swallowing amniotic fluid, the liquid that surrounds and nourishes them in the womb, according to research cited in the recent study.

Prior to the study, participants were told not to consume anything carrot- or kale-flavored on observation day, and to avoid food and flavored drinks for an hour before their scans. 

Then, researchers gave 35 of the participating women capsules of powdered carrot, and 34 got capsules of powdered kale. Thirty women didn't consume either vegetable as a control.

The team chose to administer powdered vegetables to ensure that the flavors weren't diluted during digestion — and because many pregnant women couldn't handle the taste of kale juice, Nadja Reissland, a co-author of the study, told NBC.

Past studies have documented how newborns respond to flavors that were introduced before birth in amniotic fluid, or shortly after birth in breast milk. But while prior research has considered flavor preferences post-birth, the latest study is the first to capture how fetuses react to foods ingested by the mother while they are in the womb.

High-resolution imaging allowed researchers to see fetal facial expressions

The researchers used 4D ultrasound imaging to observe the fetuses' facial expressions. The cutting-edge technology allowed them to capture more precise, frame-by-frame images compared to older imaging techniques.

About 20 minutes after the women swallowed the capsules, scans showed that fetuses exposed to carrot flavor responded with more facial expressions suggestive of laughter or smiling, while those exposed to kale flavor pressed their lips together in a grimace.

The ultrasound images showed facial movements similar to those of kids or adults who taste something bitter, such as raising the upper lip or frowning with the lower lip. However, that doesn't mean that the fetuses were expressing an early distaste for kale.

Speaking to NBC, Reissland said the grimaces observed in the ultrasounds may just be muscle movements in response to bitter flavor, although fetuses are known to make increasingly complex facial expressions towards the end of their time in the womb.