The growing fetus develops in a warm and watery world. Attached by the umbilical cord to the placenta, the developing baby floats within the fluid of the womb’s amniotic sac. Amniotic fluid first appears as a mere drop at about 3 weeks. The volume increases to about 1-½ tsp (7 mL) of amniotic fluid by 8 weeks, 2 tbs (30 mL) by 10 weeks, and ¾ cup (190 mL) by 16 weeks. There is an average of 3-¼ cups (780 mL) by 32-35 weeks.

The amniotic sac protects its cargo from the normal bumps and jostling of its environment and provides the space needed for the developing fetus to grow. This fluid-filled “weightless space” environment also allows, according to Susan Tucker Blackburn, in Maternal, Fetal, & Neonatal Physiology, “for the symmetric development of the face and body” as well as normal lung development, neuromuscular development, and exercise. Even when quarters get cramped in late gestation, the amniotic gym gives the fetus “something to push against during ‘practice’ activities.”

Fetal movements increase in frequency until approximately 32 weeks gestation, though most women find that the activity levels of the fetus periodically wax and wane. In the final weeks of pregnancy, activity lessens, though the reason may be more due to fetal brain development than the shortage of space in the womb. Fetal movement also subsides in anticipation of birth, thereby reducing the oxygen requirements of the fetus.

A Womb of Her Own
The womb is a cozy and stable environment, but it is rich in auditory stimuli: the sounds of the mother’s heart beat, digestive rumblings, and the soft whoosh of blood flow. Most of these sounds are patterned and rhythmic. Sounds from outside of the woman’s body, such as voices and music, also reach the fetus, but they are muted. The auditory system of the fetus is mature at roughly 20 weeks, and researchers have shown that the fetus can actually hear at 24-25 weeks. The fetus is also preparing for her own addition to the acoustic environment. At 11 weeks, the vocal cords of the larynx—the voice box—begin to form, even though the fetus will have to wait another 6 months to power this instrument with her first gulp of air.

It might be surprising that fetal eyes begin to develop so early, given the darkness of the womb, but by week 4, the developing eye is present as a tiny dot, and by week 6, the eye is distinct. Eyelids start developing at the same time, but by week 8, the eyelids start closing up and completely close up around 9 weeks. They reopen around week 26.

“Other stimuli that influence the activity and responses of the fetus and neonate,” according to Blackburn, “include maternal biorhythms and diurnal, circadian, and sleep-wake cycles.” It is exposure to these stimuli that provide the “experiences that help enhance neurologic organization and establish synaptic connections.” It is almost impossible to overstate how rapidly neurological development proceeds. The rate of production of neurons can be an astounding 100,000 new cells each minute.

The sensory groundwork for taste and touch are also being laid down. At 2 months, the fetus will respond to touch around the mouth; hands become sensitive to touch by 10-11 weeks. Taste receptors are present by 16 weeks and odor molecules in amniotic fluid will stimulate the fetal smell receptors by the third trimester (studies have shown that injecting a sweet substance into the amniotic sac increases fetal swallowing, whereas a bitter substance decreases swallowing). Researchers believe early exposure to such “tastes” may play a role in a baby’s later dietary preferences.

A baby will be born with millions of sensory cells that will equip her well to see, hear, and taste her new environment. But she has already had a sneak peek at the world thanks to mom. Even within the protected seclusion of the womb, the fetal environment cannot be kept separate from the maternal environment.

More on this topic

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Mother-Baby Bond: The Biology of Love
Establishing New Life
Nurture & Protect
Growth & Sensations
Adapting & Anticipating
Joyous Mom, Joyous Baby
Your Baby Enters the World
Nourishing Body & Bond
Mother & Teacher
Nurturing Development
A Mutual Gaze
From Bump to Bundle

Related Health Centers:

Infant Nutrition Health Center, Mother-Baby Bond Health Center, Mother’s Milk Health Center, Monthly Infant Development Calendar Health Center,Weekly Pregnancy Calendar Health Center