The second trimester, spanning weeks 13 through 27 of pregnancy, is when a fetus transforms from a lime-sized form with transparent skin into a nearly two-pound baby that can hear, respond to touch, and practice breathing movements. This period covers an enormous range of development: bones harden, the brain undergoes rapid growth, the lungs begin preparing for life outside the womb, and the senses come online one by one.
Bone and Muscle Development
At the start of the second trimester, around week 13, the skeleton begins hardening. Up to this point, the framework is mostly soft cartilage. Hardening starts in the skull and the long bones of the arms and legs, then continues throughout the trimester. By week 14, the neck is more defined and the head can sit more upright. By week 16, limb movements become coordinated enough to show up on ultrasound, though they’re still too subtle for you to feel.
Most people first feel fetal movement between weeks 16 and 24. If this is your first pregnancy, you’re more likely to notice it after week 20. Early movements often feel like flutters or bubbles. As muscle and bone development progresses, those movements become unmistakable kicks and rolls.
Rapid Brain Growth
The second trimester is one of the most intense periods for brain development. A structure called the subplate forms, which acts as a temporary scaffolding for the brain’s wiring. Neurons multiply rapidly in production centers and then migrate outward to their permanent positions in the brain’s outer layers. Early synaptic connections begin sprouting, and nerve fibers extend to link different brain regions together. This explosion of neural activity drives significant overall growth in head size and brain complexity.
By the end of the trimester, at week 27, the nervous system is continuing to mature but is far from finished. That maturation will carry well into the third trimester and beyond birth.
The Senses Develop in Sequence
Fetal senses activate in a specific order: touch comes first, followed by hearing and then vision. Touch sensitivity is already present early in the second trimester, and by weeks 21 to 27, fetuses respond to a mother touching her abdomen with increased arm, head, and mouth movements, along with changes in heart rate.
Hearing develops enough by around week 25 that fetuses can respond to specific sounds. In one study, 25-week-old fetuses opened their mouths more frequently when their mothers sang the syllable “LA” in a nursery rhyme compared to other stimulations. The fetus is immersed in a rich sound environment: the mother’s heartbeat, digestive sounds, and voice all reach the womb clearly.
Vision is the last sense to develop. The eyes begin moving slowly around week 16, and the eyelids remain fused shut until about weeks 23 to 24. Even before the eyes open, touching the closed eyelids may generate flashes of light because the retina is already well developed at that stage. By week 26, the eyes are fully developed structurally, though they may not open for another couple of weeks. Research has shown that fetuses around 25 weeks can even distinguish face-like patterns of light shone through the uterine wall, preferring upright face shapes over inverted ones.
Lungs Begin Preparing to Breathe
The lungs undergo critical development during the second trimester, though they won’t be ready to function independently until much later. Around week 20, specialized cells in the lungs start producing tiny storage packets of surfactant, a slippery substance made of fats and proteins that keeps the air sacs from collapsing. Surfactant production begins in earnest at week 24, but adequate amounts to support breathing outside the womb aren’t produced until around week 32, well into the third trimester.
This timeline is why premature birth during the second trimester is so dangerous. Survival rates reflect how far lung and organ development has progressed: at 22 weeks, survival is roughly 3 to 8 percent. At 24 weeks, it rises to about 24 to 46 percent depending on the population studied. By 26 weeks, survival reaches 58 to 79 percent. Each additional week in the womb makes a significant difference.
Skin, Hair, and Protective Coatings
At week 13, the skin is paper-thin and translucent. Over the following weeks it thickens steadily. Around three months into development, a fine layer of hair called lanugo covers the body. This hair isn’t just incidental. It serves as an anchor for vernix caseosa, the waxy, white coating that covers the fetus. Vernix protects the skin from the amniotic fluid, which contains urea and electrolytes that would otherwise be irritating over months of constant exposure. It also helps with temperature regulation and provides a barrier against infection.
Without lanugo holding the vernix in place, the coating would simply slide off the skin and lose its protective function. Most of this fine hair is shed before birth, though premature babies are often still covered in it.
By week 26, eyebrows and eyelashes have formed, and by week 27, the fetus starts gaining a layer of fat beneath the skin. This fat smooths out the previously wrinkled appearance and will become increasingly important for temperature regulation after birth.
Sex Can Be Identified on Ultrasound
External genitalia begin forming around week 10 but aren’t developed enough to see on imaging at that point. By week 14, external genitals are fully formed and fingerprints start developing. The anatomy scan, typically performed around week 20, is when most people learn the sex of their baby. This ultrasound also checks the physical development of organs, limbs, and the spine.
Blood Production and Organ Maturation
At week 14, the spleen begins producing red blood cells, taking over a job that was previously handled by other temporary structures. The fetus at this point is about 3.5 inches long and weighs roughly 1.5 ounces. By week 16, it has grown to nearly 5 inches and 4 ounces.
Growth accelerates dramatically through the rest of the trimester. By week 26, the fetus measures about 9 inches from crown to rump and weighs close to 2 pounds, roughly 18 times heavier than at week 14. The digestive system, kidneys, and other organs are maturing throughout this period, though most won’t be fully functional until the third trimester or after birth.
Sleep and Activity Patterns
By the later weeks of the second trimester and into the third, fetuses begin cycling between quiet (sleep-like) and active states. During quiet periods, heart rate is steady and movement is minimal. During active periods, the heart rate shows more variability and the fetus moves frequently. These cycles vary widely between individual fetuses. In recordings of normal fetuses, the median time spent in a quiet state was about 26 percent of a one-hour observation, but this ranged from 0 to nearly 89 percent, showing just how much natural variation exists. As pregnancy advances, the active periods show increasingly frequent heart rate accelerations, a sign of a maturing nervous system.