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