Fetal Development During the Second Trimester
Week 13: Skeleton Strengthens
At the start of the second trimester, the baby’s bones begin to firm up. The skull and long bones show this change first. Though the skin is still very thin and almost see-through, these early developments help shape the baby’s body. Organs, muscles, and the basic nervous system start to work together.
Week 14: Making Red Blood Cells
By this week, the baby’s neck becomes more defined, and the spleen starts making red blood cells. The baby grows to about 3 and a half inches long and weighs close to 1 and a half ounces. It might become possible to see if the baby is a boy or girl during an ultrasound. Fetal growth picks up speed as more blood cells form and the body continues to mature.
Week 15: Growing Bones and Hair Patterns
Bones keep growing and appear even clearer on ultrasounds. Not only do bones get stronger, but the scalp also develops a pattern where hair will grow. The brain and nervous system start to guide body movements.
Week 16: Moving Eyes and Limbs
The baby’s head now sits upright and the eyes begin small movements, showing early brain and nervous system development. The ears shift into their final spots, and the skin thickens. Movements of arms and legs become more coordinated, though these motions might still be too light for a parent to feel. The fetus measures nearly five inches and weighs about four ounces at this stage.
Week 17: Toenails Appear and Activity Increases
At this stage, tiny toenails start to form. The baby becomes more active, rolling and turning inside the uterus. Hiccups may begin, leading to small, rhythmic jerks that sometimes parents can feel. The nervous system keeps getting more complex, preparing for future movement and responses.
Week 18: Beginning to Hear
This week, the baby shows even more progress. The ears now stick out from the head and the baby starts to hear sounds, such as the mother’s heartbeat or voice. The digestive system begins working, getting ready for life after birth. Now, the fetus is about five and a half inches long and weighs around seven ounces. This early ability to hear marks another step in nervous system development.
Table: Week 18 Highlights
| Feature | Description |
|---|---|
| Ear position | In final location |
| Hearing | Begins to sense sounds |
| Digestive system | Starting to function |
| Fetal size | ~5.5 inches, 7 ounces |
Week 19: Protective Coating Forms
A layer called vernix caseosa starts to cover the baby’s skin. This coating is greasy and white and shields the delicate skin from amniotic fluid. Around this time, the baby also starts releasing urine, which becomes a big part of the surrounding amniotic fluid. Growth slows for a bit but continues steadily.
Week 20: Midway Milestone
The pregnancy now hits the halfway point. Many mothers notice the baby’s first movements, called quickening, at this time. The baby follows a regular sleep-wake cycle and can be awakened by sudden noises. By week 20, the baby measures roughly six and a third inches and weighs about eleven ounces.
Sleep-Wake Patterns
- Baby begins developing cycles of activity and rest.
- External noises can wake the baby.
- Movements are easier to notice.
Week 21: Sucking Reflex and Lanugo
Fine, soft body hair called lanugo now covers the baby’s body. This helps the protective vernix caseosa stay in place. The sucking reflex starts to appear, and the baby might suck on a thumb inside the womb. These changes are early signs of important skills needed after birth.
Week 22: Eyebrows and Head Hair Grow
Small hairs above the eyes and on the scalp become visible this week. In boys, the testes move toward the scrotum, while in girls, the uterus and ovaries are in place and the vagina forms. The baby is now about seven and a half inches from head to rump and weighs around one pound.
Visual Chart: Sex Development
| Feature | Males | Females |
|---|---|---|
| Glands | Testes move downward | Uterus and ovaries in place |
| Hair | Eyebrows, scalp hair grow | Same |
Week 23: Finger and Footprints Develop
Unique ridges begin to show up on the baby’s palms and soles. These will later form fingerprints and footprints. The baby begins rapid eye movement, showing that the sleep cycle is developing. The lungs now create surfactant, a substance that helps the air sacs expand and not stick together, which is important for future breathing. Babies born this early sometimes survive with help.
Fetal Development Timeline
- Surfactant production starts.
- Eye movement increases.
- Fingerprint and footprint ridges visible.
Week 24: Wrinkled, Transparent Skin
The baby’s skin looks wrinkled because there is still little body fat. The skin is quite see-through, and blood vessels can be seen underneath, making it appear pink or red. Weight gain is steady—the fetus now weighs over one and a third pounds and measures more than eight inches.
Week 25: Reaction to Sounds
From week 25, the baby moves in response to familiar voices or outside noise. The sleep cycle now often includes rapid eye movement (REM), important for brain and nervous system growth. This sleep stage is similar to newborn and adult sleep patterns.
Quick List: Week 25 Milestones
- Baby moves to noises and voices.
- Eyes show REM sleep pattern.
- Brain activity increases.
Week 26: Eyebrows and Lashes Finish Forming
Eyebrows and eyelashes are fully developed. The eyes are well-formed but may not open for another couple of weeks. The baby now measures about nine inches from crown to rump and weighs almost two pounds. Sensory development moves fast, with the nervous system helping the baby respond to touch and sound.
Week 27: Closing the Second Trimester
By the end of week 27, the second trimester comes to a close. The baby’s nervous system matures, helping control movement and other body functions. Fat layers build up, smoothing out the wrinkled skin. Growth in weight and size continues, preparing the baby for the final trimester before birth.
Table: Summary of Major Second Trimester Changes
| Week | Main Developments | Size and Weight |
|---|---|---|
| 13 | Bones start to harden | |
| 14 | Red blood cells form in spleen | ~3.5 in, ~1.5 oz |
| 15 | Bone and scalp hair pattern form | |
| 16 | Eyes move, limbs coordinate | ~4.75 in, ~4 oz |
| 17 | Toenails, more movement | |
| 18 | Hearing develops, digestive system works | ~5.5 in, ~7 oz |
| 19 | Vernix coating forms | |
| 20 | Movement felt, sleep cycles begin | ~6.3 in, ~11 oz |
| 21 | Sucking reflex, lanugo grows | |
| 22 | Eyebrows, hair, sex organs develop | ~7.5 in, ~1 lb |
| 23 | Fingerprints, surfactant in lungs | |
| 24 | Skin wrinkled and pink | ~8.25 in, ~1.3 lb |
| 25 | Responds to voices, REM sleep pattern | |
| 26 | Eyebrows and eyelashes formed | ~9 in, ~2 lb |
| 27 | Nervous system matures, fat layers increase | Growing steadily |
