Human Body

Reproduction and Human Development

What are some major developmental events during the second trimester of pregnancy?

The second trimester of a pregnancy lasts from weeks 13 through 27. Each week brings changes and new developments in the fetus.

Major Developmental Events during the Second Trimester
Time Period Major Developments
Week 13 Baby begins to move, although the movements are too weak to be felt by the mother; ossification of bones begins
Week 14 Prostate gland develops in boys; ovaries move from the abdomen to the pelvis in girls
Week 15 Skin and hair (including eyebrows and eyelashes) begins to form; bone and marrow continues to develop; eyes and ears are nearly in their final location
Week 16 Facial muscles are developing allowing for facial expressions; hands can form a fist; eggs are forming in the ovaries in girls
Week 17 Brown fat tissue begins to develop under the skin
Week 18 Fetus is able to hear such things as the mother’s heartbeat
Week 19 Lanugo and vernix cover the skin; fetal movement is usually felt by the mother
Week 20 Skin is thickening and developing layers; fetus has eyebrows, hair on the scalp and well-developed limbs; fetus often assumes the fetal position of head bent and curved spine
Week 21 Bone marrow begins making blood cells
Week 22 Taste buds begin to form; brain and nerve endings can process the sensation of touch; testes begin to descend from the abdomen in boys; uterus and ovaries (with the lifetime supply of eggs) are in place in girls
Week 23 Skin becomes less transparent; fat production increases; lungs begin to produce surfactant, which will allow air sacs to inflate; may begin to practice breathing
Week 24 Footprints and fingerprints begin to form; inner ear is developed, controlling balance
Week 25 Hands are developed, although the nerve connections are not yet fully developed
Week 26 Eyes are developed; eyebrows and eyelashes are well-formed; hair on head becomes fuller and longer
Week 27 Lungs, liver, and immune system are developing


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