Weeks 15 & 16
Stress response and pain
- PCW 15&16iPost-Conception Weeks
- 5 Months Pregnant
- Gestational Weeks 17&18
The fetus responds to a needle entering his hepatic vein 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
About 15 weeks after conception, fetal blood cells start forming in the bone marrow.4 Blood cells will form in the bone marrow for the rest of the baby’s life.
Although the baby started moving 6 weeks after conception, 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 after conception if she has had a previous child, and 18 weeks if she has not. This is also known as quickening.5
These weeks, the airways in the lungs keep developing. Cilia, which started to grow 13 weeks after conception, 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
Some experts believe that the formation of airways in the lungs finishes about 16 weeks after conception,8 although the sites of gas exchange in the lungs, called alveoli, still need to develop and mature.
Furthermore, sporadic eye movements start between 14 and 16 weeks after conception.9 These have been detected using ultrasound.
Finally, stress and pain responses in the unborn have been recorded from fetuses as young as 16 weeks after conception. When doctors used a need to draw blood from a vein near the fetus’s liver, the fetus responded by recoiling. Furthermore, 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.10