Describe major prenatal milestones during the second trimester of pregnancy

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Prenatal development is sometimes divided into trimesters, or three equal time periods. The second trimester starts at the beginning of the fourth month. At the beginning of the second trimester, the external genitals are well-formed, and the sex of the fetus can be detected with ultrasound. Other finishing touches appear, such as fingernails, toenails, tooth buds, and eyelids. The heartbeat can be heard through a stethoscope.
By the middle of the second trimester, between 17 and 20 weeks, the new organism has grown large enough that the mother can feel its movements. A white, cheeselike substance called vernix protects its skin from chapping during the long months spent bathing in the amniotic fluid. White, downy hair called lanugo also appears over the entire body, helping the vernix stick to the skin.
At the end of the second trimester, many organs are well-developed. Most of the brain's billions of neurons are in place; few will be produced after this time. However, glial cells, which support and feed the neurons, continue to increase rapidly throughout the remaining months of pregnancy, as well as after birth. At the same time, neurons begin forming synapses, or connections, at a rapid pace. Brain growth means new behavioral capacities. The 20-week-old fetus can be stimulated as well as irritated by sounds. And if a doctor looks inside the uterus using fetoscopy, fetuses try to shield their eyes from the light with their hands, indicating that sight has begun to emerge. Still, a fetus born at this time cannot survive. Its lungs are immature, and the brain cannot yet control breathing and body temperature.