Charlotte Lozier Institute

Phone: 202-223-8073
Fax: 571-312-0544

2776 S. Arlington Mill Dr.
Arlington, VA 22206

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Weeks 17 & 18

Stress response and sensations

Human Prenatal Age
  • Post-conception week 15&16
  • 5 Months Pregnant
  • Gestational Weeks 17&18
  • The fetus responds to a painful procedure with a stress response that includes recoiling and increasing circulating stress hormones in his bloodstream separate from any stress hormones in the mother’s bloodstream.1

  • The fetus begins storing energy as body fat. Fat deposits make the baby’s cheeks chubby.2

  • The corpus callosum, which connects the right and left sides of the brain, begins developing.3

Although the baby started moving at 7 ½ weeks gestation, the fetus has finally become big enough and strong enough for the mother to feel. A pregnant mother can usually feel her unborn child moving around 16 weeks if she has had a previous child, and 18 weeks if she has not. This is also known as quickening.4

In week 17 and 18, the mother may feel her unborn child moving. (Image Credit: Priests for Life)
The fetal stress response

Stress and pain responses in the unborn have been recorded from fetuses as young as 18 weeks gestation. When doctors used a needle to draw blood from a vein near the fetus’s liver, the fetus responded with “vigorous body and breathing movements” similar to recoiling. Furthermore, five minutes after the transfusion, large increases in stress hormones were observed in the fetus’s blood, but not the mother’s blood, showing that the fetus’s stress response was separate from the mother’s. Additionally, when a needle was inserted into the umbilical cord, which does not have pain receptors, the fetus did not respond with a large increase in circulating stress hormones. This shows that the hormones are released in response to the sensation of pain.5

Lung development

Additionally, the airways in the lungs keep developing. Cilia, which started to grow at 15 weeks, are now developing at a rapid pace.6 Cilia are tiny hair-like projections protruding from the cells that line the airway. Cilia catch debris and mucus and sweep it towards the throat, helping prevent infections in the lungs. The mature cells that line the airway will have about 200 cilia each.7

Airways in the lungs finish forming around 18 weeks,8 although the sites of gas exchange in the lungs, called alveoli, still need to develop and mature.

When does a baby start forming blood?

Although the baby has blood by week 6 when the heart starts beating, many of the complex cells in the blood, such as red and white blood cells form later and in different locations than they do in the adult. For example, around week 11, red blood cells start forming in the liver.9 At 17 weeks, fetal blood cells start forming in the bone marrow.10 Blood cells will form in the bone marrow for the rest of the baby’s life.

Dive Deeper
The old, uninformed notions that unborn and newborn babies cannot feel pain are refuted by a growing body of scientific evidence.
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