A frequent scene in sitcoms or movies involves a romantic sex scene with a man and a pregnant woman. The man suddenly becomes concerned that he will poke the baby with his penis during sex with a pregnant partner. Hilarity ensues. Don’t believe me? Go watch Knocked Up again. Spoiler alert: a penis won’t poke the baby during sex. But, that doesn’t mean your baby isn’t experiencing input from their environment. Your baby can experience lots of other things during their time in utero – sounds, taste, touch, and even sights. Let’s discuss how each develops in turn.
Although it is obviously dark in the womb, a baby’s visual system is still developing and getting input. About 4 weeks after conception, cells form in the developing brain that will form the optic nerves behind each eye. Over the next few weeks, clusters of other cells will form the retina, lens, and remaining parts of the eyes. Sometime around 4 months, a baby’s eyes can respond to light and dark and will shift to stimuli. Note though that the baby still has not opened his or her eyes. Late in the second trimester, a baby can finally open his or her eyes and the eyes are more or less fully developed.
Generally, the visual stimulation in utero is limited to patches of light and dark that comes filtered through the uterus walls. This is one of the reasons why visual stimulation can be very overwhelming for a newborn – they have never seen so much! Some babies seem especially sensitive to the new experiences of visual stimulation outside the womb, which is why retreating to a dark room can sometimes calm an upset or over-stimulated newborn.
The sense of touch develops in the first few months of pregnancy. The first place that touch receptors develop in on the face, generally around the nose and lips (around 8 weeks), followed by hands, feet, and genitals (around 12 weeks), and the abdomen (around 16 weeks). By the third trimester, a baby is covered in touch receptors all over his or her body.
In utero, nobody is directly touching or cuddling the developing fetus, yet the fetus is still getting quite a bit of tactile stimulation. As the baby moves around, it comes into contact with their own body and the walls of the uterus. Indeed, ultrasounds have revealed babies sucking their thumbs, touching their face or chest, and even playing with their umbilical cord. Most of the tactile stimulation later in pregnancy (when quarters become quite cramped) is by a baby putting their hands in their mouth. It’s no surprise then that this is a typical behavior observed among newborns – more evidence of continuity of child development from utero to the outside world.
In addition to touching him or herself, a fetus also gets tactile stimulation from touching the walls of the uterus. Earlier in pregnancy, a fetus can float around pretty freely in the uterus, but as the space becomes more cramped, a fetus is frequently touching and stretching out – pushing up against the walls of the uterus and a woman’s bladder.
Twins, or other multiple-fetus pregnancies, spend quite a bit of time touching each other. Indeed, outside of utero, twins who are touching each other experience a more stable and regular heartbeat. Fascinatingly, if one twin is having a difficult time managing their heart rate after birth, placing this twin so that he or she can touch their twin improves breathing and heart rate in the weaker of the two. Because of this, most hospitals now allow twins to share a bassinette. The early tactile stimulation from utero seems to carry forward protective benefits on the twins outside of utero.
Remember that at the end of pregnancy, a baby is very cramped in the uterus. This is why newborns tend to prefer to be swaddled and held: the touch sensation mimics what they experienced in utero.
Taste & Smell
You might think that a baby can’t possibly have a developed sense of taste before they try milk, but that is actually not the case. Amniotic fluid, which a baby is swallowing while in utero, contains a variety of flavors.  Where do these flavors come from? From what a pregnant woman is eating.
Beyond tasting different sorts of flavors, babies also have a preference for sweet over savory, a preference appears to emerge in utero. In a strange study from the mid 20th century, a researcher (Dr. DeSnoo) devised a way to help women with excess amniotic fluid: he injected saccharin (sugar) into the amniotic fluid. Turns out, fetus’ drank more amniotic fluid when it had been sweetened, thus reducing the amount of amniotic fluid. This suggests that even prior to birth, babies have a preference for sweeter taste. This preference serves babies well, as human breast milk is very high in lactose (milk sugar), making it quite sweet.
Related to the sense of taste is the sense of smell. In utero, babies aren’t exactly smelling flowers, but it is true that amniotic fluid takes on odors from what the mother has eaten. Usually, these odors are from strongly flavored foods, like coffee, garlic, turmeric, or onion. In fact, many OB/GYN’s report that when a woman’s water breaks during labor, you can smell differences based on recent food consumed by the mom. Turns out, smells can be translated through liquid into the olfactory receptors – so when a baby is tasting amniotic fluid, they are also smelling it. It all makes sense if you remember the childhood trick of plugging your nose so you can’t taste the disgusting vegetables on your plate.
The cells that will form the ears and auditory system begin developing 4 weeks after conception. By the 6th month of prenatal development, and possibly sooner, a fetus can hear and will respond to sounds happening in the outside world (i.e., the mom’s environment). Using changes in heart rate and fetal movement as markers of a fetus hearing external sounds, we know that about 6 months after conception a fetus’ hearing system is developed enough to respond to changes in sound. By the end of pregnancy (roughly after 38 weeks), changes in heart rate patterns suggest that a fetus can distinguish between music and speech. The baby can also seem to tell the difference between a mother’s voice and other voices.
These preferences are lasting – with newborn babies preferring to listen to their own mother’s voice, the languages they heard in the womb compared to a different language, and to stories that were read to them in utero.
Although people have long hypothesized that what a child hears in utero may have effects on intelligence, there is no evidence that this is true.
However, it is true that sounds heard in utero will be familiar to a newborn and possibly provide some comfort. Moreover, muffled sounds like white noise can also be calming to an infant. This is likely because sound heard in the uterus is dulled and muffled as it travels through the amniotic fluid. As many parents can attest, loud, dulled white noise (like the dryer or vacuum), can be life change for settling down a crying newborn. As with the other senses, mimicking the sensations of the prenatal period can be a powerful help in soothing a fussy newborn baby.