Smell and taste in the womb

The fetal nose starts detecting smell molecules around 7 months gestation. But before the fetus can smell, the appropriate structures and circuits must develop first. Smell-sensing cells, called olfactory receptor neurons, are made to detect over 1 trillion different smells in humans.¹ Located in the top of the nasal cavity, olfactory receptors bind to smell molecules. These olfactory receptor neurons start growing in week 11 and have a mature shape by week 13.^{2 3} When the olfactory receptor neurons detect a smell, it sends an electrical signal to the olfactory bulb in the brain. The olfactory bulb starts forming in week 9^{4 5} with a distinct layered structure in place by week 14.⁶

Thousands of olfactory receptor neurons that detect the same type of smell connect to a single neural cluster called a glomerulus. Each unique glomerulus corresponds to distinct type of odor, forming many glomeruli for the detection of many smells. While thousands of olfactory receptor neurons may synapse on one glomerulus, only about 25 mitral relay cells send outputs from that glomerulus to the rest of the brain.⁹ The mitral cells start forming by week 11.¹⁰

By 14 weeks, most of the structures needed for smell have formed, but clear evidence of smell processing only appears around 28 to 30 weeks.¹¹ Around that time, scientists detect olfactory marker protein. This protein is a strong indicator that smell receptors are mature, functional, and ready to process smells in the baby’s environment.^{12 13} Another nerve, called the trigeminal nerve—which senses irritation and touch in the nose—is active much earlier, around 8 to 10 weeks, and may respond to certain chemicals before true smell sensation has fully developed.¹⁴

Between 28 and 30 weeks, unborn babies begin smelling through the amniotic fluid around them. ¹⁵ The fluid carries a mix of scents that come from the baby’s own body and from the mother’s diet.¹⁶ Studies of premature infants show that by about 29 weeks, babies change their breathing, alertness, and facial expressions in response to different odors. Pleasant smells like vanilla increase the baby’s breathing rates, while unpleasant odors such as rancid acid or disinfectant, trigger slower breathing rates and grimacing, even in small amounts.^{17 18} By 7 months preborn babies can react to a variety of scents.¹⁹

The taste system starts coming together early in pregnancy.  Starting at 8 weeks, small bumps called papilla start forming on the tongue.³¹ Taste papilla are small ridges or mushroom-shaped bumps that house taste buds and increase surface area to enhance taste perception. In week 8, nerves start entering the tongue.³² These nerves are especially important for causing taste buds to form.³³ In weeks 9 and 10, early taste buds start developing.³⁴ Mushroom-shaped papillae start growing.³⁵ In week 11, about one quarter of early taste buds are connected to nerve fibers.³⁶ Two different types of taste bud cells can be observed on the fetal tongue by 14 weeks.³⁷ Taste pores, commonly acknowledged as a sign of taste bud maturity, appear between weeks 12 and 16.³⁸ By week 16, the inside of the taste pore, called the taste pit, is full of microvilli, tiny fibers that interact with taste molecules. Most of the taste buds have connected to nerves, and the fetus may be able to taste. By 17 weeks, fetal taste buds resemble adult taste buds in shape if not size.³⁹

The fetus gets lots of taste experience before birth. Flavors from the mother’s food, like garlic, spice, or kale change the flavor of the amniotic fluid after a meal.⁴¹ By 34 weeks, unborn babies will swallow less amniotic fluid if the amniotic fluid tastes bitter,⁴² and more if it tastes sweet.⁴³ Another recent study showed that fetuses this same age make more “laughter-like” facial expressions after their moms eat a sweet carrot flavor, while those exposed to kale made more “cry-like” faces.⁴⁴