Weeks 17 & 18

Breathing, crying, and the rooting reflex

Human Prenatal Age
  • PCW 17&18
    i
  • 5 Months Pregnant
  • Gestational Weeks 19&20
Highlights
  • Multiple research studies have shown that the fetus responds to painful stimuli with a stress response that includes recoiling and increased circulating stress hormones separate from stress hormones in the mother’s bloodstream.1

  • The skin has developed sweat glands.2

  • By 17 weeks after conception, the fetal heart has beat over 20 million times.3

  • The fetal heart pumps about 55 quarts of blood each day.4

The fetus practices lots of activities he will use outside the womb. For example, by 18 weeks after conception, the preborn baby’s vocal cords move in a similar pattern to that seen during crying after birth.5 The fetus also practices breathing.6

Although the fetus used to move away when something touched his face, the fetus now rotates his head towards things that touch his face while simultaneously closing his mouth.7 These movements mark the start of the rooting reflex, which helps a newborn find his mother’s nipple during breastfeeding.

7
milliliters
The fetus drinks 2-7 mL of amniotic fluid each day.8 For comparison, the fetus drinks 500-1000 mL of amniotic fluid each day in the last month of pregnancy.9 Similarly, about a week after birth a newborn drinks about 500 mL of breastmilk or formula per day.10
This baby is already practicing breathing, crying and breastfeeding to prepare for his birth. (Image Credit: Priests for Life)

Although the eyelids are closed, the eyes keep developing. The back of the eye, called the retina, which converts light into neural signals, now has discrete layers, separating cell types.11

In animals, waves of spontaneous electrical activity cross the retinas. This allows the retinal cells to properly connect to the visual cortex in the brain, and to organize inputs from the right and left eyes.12 Although these spontaneous retinal waves have never been directly measured from the human fetus, they likely begin around this age.

Dive Deeper
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