Weeks 17 & 18

By 17 weeks of pregnancy, the baby’s entire body is sensitive to light touch, with the possible exception of the inner thighs.⁷ The baby can make a variety of facial expressions including a squint, scowl, and sneering motion. ⁸ In ultrasounds, the baby has been seen smiling as early as 15 weeks, and the number of smiles increase in frequency at 18 weeks .⁹ The unborn baby will also grasp objects that come near him using all five fingers.¹⁰ Furthermore, some babies stretch upright with crossed legs, striking poses that resemble yoga.¹¹ Even in these early weeks, their movements are varied, expressive, and surprisingly human.

The thin tissue that plugs the nostrils during early development dissolves by week 18, allowing amniotic fluid to bathe the inner nostrils of the unborn baby.¹² As the baby practices breathing in the womb, amniotic fluid can now circulate through his nose. This allows chemicals in the amniotic fluid to interact with the young smell receptors.¹³ It remains unknown how much the fetus can smell before 28 to 30 weeks.

By 17 weeks, taste buds have reached their mature shape and the taste receptor cells have microvilli — microscopic fibers specialized for detecting food molecules.¹⁴ The microvilli on the taste receptor cells extend into the taste pore to interact with food molecules.¹⁵ These mature receptors make it possible for the fetus to detect tastes in the amniotic fluid.¹⁶

By the last week of 4 months of pregnancy—17 weeks, the main fingerprint patterns—whorls, loops, and arches—have already formed, and smaller ridges begin to develop between them.³²

Between 17 and 23 weeks, the unborn baby displays a wide range of eye activity, including sustained gazes and rapid shifts followed by repositioning movements.³³ Scientists can use eye motion to tell whether the fetus is awake, in quiet sleep, or in rapid-eye movement sleep, when the unborn baby might be dreaming.³⁴ However, rapid-eye movements are usually only detected after 23 weeks.³⁵