The second trimester of pregnancy spans week 13 through the end of week 27, covering roughly the fourth, fifth, and sixth months. It’s often called the most comfortable stretch of pregnancy: first-trimester nausea typically fades, energy returns, and the baby undergoes dramatic growth. During these 15 weeks, your baby develops from a lemon-sized fetus with a soft skeleton into a nearly two-pound baby that can hear your voice and respond to it.
How Your Baby Develops Week by Week
At the start of the second trimester, bones in the skull, arms, and legs begin to harden. By week 14, the spleen is producing red blood cells, and by week 15, a scalp hair pattern is forming. These early weeks lay the structural groundwork for everything that follows.
The middle stretch, roughly weeks 16 through 21, is when the senses come online. Around week 16, your baby’s eyes can move slowly and the ears are nearly in their final position. By week 18, the digestive system is working and your baby may begin to hear sounds. A protective waxy coating starts covering the skin around week 19, and the baby begins releasing urine that makes up most of the amniotic fluid from this point forward. By week 21, a fine layer of downy hair covers the entire body, and the sucking reflex is developing, so thumb-sucking may begin.
The final weeks of the trimester focus on refining what’s already in place. Around week 23, ridges form on the palms and soles that will become fingerprints and footprints. The lungs start producing a substance that allows air sacs to inflate properly after birth. By week 25, your baby responds to familiar sounds, especially your voice, and spends most sleep time in rapid eye movement. Eyebrows and eyelashes are fully formed by week 26. By week 27, the nervous system is maturing rapidly and the baby is gaining fat, which smooths out the previously wrinkled, translucent skin.
What Changes in Your Body
Your body is working significantly harder than it was before pregnancy, even if you feel better than you did in the first trimester. Total blood volume increases by roughly 45% over prepregnancy levels, with plasma volume rising faster than red blood cell production. This imbalance creates a mild “physiological anemia” from dilution, which is normal. Your heart pumps more blood per minute, and your lungs take in more air with each breath to meet the increased oxygen demands of both you and the baby.
The uterus, which started the pregnancy roughly the size of a pear, rises above the pelvis during the second trimester and becomes visibly prominent. This is when most people start “showing.” You’ll likely feel the baby’s first movements, called quickening, somewhere between weeks 16 and 22. Early on it feels like fluttering or bubbling; by the end of the trimester, kicks and rolls are unmistakable.
Other common changes include round ligament pain (sharp twinges in the lower belly as the uterus stretches), nasal congestion from increased blood flow to mucous membranes, and skin changes like a dark line running down the abdomen. Many people also notice their gums bleeding more easily when brushing, which is another effect of the extra blood volume.
Key Screenings and Tests
Two major medical appointments typically happen during the second trimester: the anatomy scan and the glucose screening.
The anatomy scan is an ultrasound performed around week 20. A sonographer takes detailed pictures and measurements of your baby’s heart, brain, spine, kidneys, bladder, lungs, stomach, intestines, limbs, and facial features. They also record the fetal heart rate, check blood flow through the umbilical cord, note the position of the placenta, measure amniotic fluid levels, and examine your cervix. This is the appointment where you can typically learn the baby’s sex if you want to know. If any measurements fall outside the expected range, your provider may recommend follow-up imaging.
Between weeks 24 and 28, most pregnant people are screened for gestational diabetes with a glucose challenge test. You drink a sugary solution, and your blood sugar is measured an hour later. A result of 140 or higher means you’ll need a longer fasting version of the test to confirm whether gestational diabetes is present. A reading of 200 or above may indicate pre-existing type 2 diabetes rather than the gestational form.
Sleep Position and Daily Comfort
As the uterus grows, sleeping on your back can press its weight against the large vein that returns blood from your lower body to your heart. Researchers have studied whether this compression affects pregnancy outcomes. An NIH-funded study found that sleep position during early and mid-pregnancy does not appear to affect the risk of complications. As the second trimester progresses, many people naturally find side sleeping more comfortable simply because lying flat becomes awkward. A pillow between the knees or under the belly can make a noticeable difference.
Warning Signs to Watch For
Most of the second trimester passes without serious complications, but a few conditions can emerge during this period. Preeclampsia, a blood pressure disorder, typically develops after 20 weeks. Its hallmark is new high blood pressure, but it can also cause severe headaches, vision changes (blurriness, light sensitivity, or temporary vision loss), upper belly pain concentrated under the right ribs, nausea, shortness of breath, and sudden swelling of the face or hands. Sudden, significant weight gain can also be an early signal.
Signs of preterm labor before week 37 include regular contractions that don’t stop when you change position, persistent low back pain, pelvic pressure, and a change in vaginal discharge. Leaking fluid, vaginal bleeding, or a noticeable decrease in fetal movement after you’ve been feeling regular kicks are all reasons to contact your provider promptly rather than waiting for a scheduled appointment.