Second Trimester Week by Week: Pediatrician’s Guide (2026)
Second Trimester Week by Week: Your Complete Guide to Weeks 13–27
A week-by-week breakdown of fetal development, maternal changes, and what to expect during the second trimester — the most comfortable stretch of pregnancy for most women.
Second Trimester Overview: What Changes and When
The second trimester spans weeks 13 through 27 of pregnancy, and many women describe it as the “honeymoon phase” for good reason. Nausea typically subsides, energy levels improve, and the risk of miscarriage drops significantly after week 13 (American College of Obstetricians and Gynecologists, 2023). Your baby grows from roughly the size of a lemon to a full-term cauliflower, developing every major organ system along the way.
Understanding the progression week by week helps parents prepare for each stage. During the second trimester, fetal movement becomes detectable, sex can often be determined via ultrasound, and the brain undergoes its most rapid period of growth. For mothers, the expanding uterus begins to show visibly, and symptoms like round ligament pain and nasal congestion become common.
Each week brings specific developmental milestones. The sections below walk through every week from 13 to 27, covering fetal size, organ development, maternal changes, and practical tips for staying healthy during this stage.
Week 13: The Transition Begins
Week 13 marks the official start of the second trimester. Your baby measures about 2.9 inches from crown to rump and weighs approximately 0.81 ounces — roughly the size of a peapod. The head still accounts for about half of the total body length, but the body is beginning to catch up.
The intestines, which had been developing in the umbilical cord, move into the abdomen this week. Fingerprint ridges start forming on the fingertips, and the vocal cords begin developing. For the mother, morning sickness usually starts fading, though it does not disappear for everyone. Fatigue may persist but often improves over the next two to three weeks.
Week 14: Hormonal Shift and Energy Return
The placenta is now fully formed and takes over hormone production from the corpus luteum. This hormonal handover is one reason nausea decreases for many women. Your baby is about 3.4 inches long and weighs 1.5 ounces. The neck is more defined, and the chin lifts from the chest. The arms lengthen and begin to look more proportional to the body.
Maternal changes include increased blood volume — up to 40–50% more than pre-pregnancy levels by the third trimester — and the beginning of a visible bump. Some women notice linea nigra, a dark vertical line on the abdomen, appearing for the first time.
Week 15: Hearing Development Starts
Your baby reaches about 4 inches in length and weighs around 2.5 ounces. The ears move into their final position on the sides of the head, and the inner ear structures mature enough to begin detecting sound. Light-sensitive eyes develop, though the eyelids remain fused.
For mothers, weight gain typically picks up now. Most providers recommend a gain of roughly 1 pound per week during the second and third trimesters for women with a pre-pregnancy BMI in the normal range (Institute of Medicine, 2009). Nasal congestion and mild nosebleeds become common due to increased blood flow to mucous membranes.
Week 16: Finding Out Sex and Round Ligament Pain
The baby weighs about 3.5 ounces and measures 4.6 inches. The musculoskeletal system matures, and facial muscles can now make expressions like squinting and frowning. Genital development is typically complete enough for ultrasound technicians to determine sex, though accuracy depends on fetal position and equipment quality.
Round ligament pain — sharp, brief stabbing sensations in the lower abdomen or groin — often begins this week as the ligaments stretch to support the growing uterus. Changing positions slowly and gentle stretching help. Some women also notice their first fetal movements, described as fluttering or gas bubbles, especially in subsequent pregnancies.
Week 17: Baby Fat Begins Forming
Your baby is 5.1 inches long and weighs about 5 ounces. Brown fat tissue starts developing under the skin. This specialized fat helps regulate body temperature after birth. The skeleton continues its transition from soft cartilage to bone through a process called ossification.
Mothers may experience increased appetite and a visible darkening of the areolas. Leg cramps, particularly at night, affect up to 30% of pregnant women and often respond to magnesium-rich foods or gentle calf stretches before bed (Ayden et al., 2019).
Week 18: The Anatomy Scan
The mid-pregnancy anatomy ultrasound typically takes place between 18 and 22 weeks. This detailed scan examines the brain, heart, spine, kidneys, stomach, and limbs for structural abnormalities. Your baby is about 5.6 inches long and weighs 7 ounces.
The heart has four chambers and beats at 140–150 beats per minute — twice the adult rate. Myelin, a fatty substance that insulates nerve fibers, begins forming around the nervous system. Mothers often report stronger, more distinct fetal movements by this week. Sleep positions should start shifting to the left side to optimize blood flow to the uterus.
Week 19: Vernix Caseosa Appears
Your baby measures 6 inches and weighs about 8.5 ounces. A waxy, cheese-like coating called vernix caseosa forms on the skin to protect it from prolonged exposure to amniotic fluid. Without it, the skin would macerate and break down. Fine hair called lanugo covers the body and helps hold the vernix in place.
The sensory organs are rapidly maturing. The baby can hear your voice, digestive sounds, and heartbeat clearly now. Some research suggests newborns recognize stories read aloud repeatedly during pregnancy (Partanen et al., 2013). For mothers, a noticeable bump is visible to others, and stretch marks may begin appearing on the abdomen, hips, and breasts.
Week 20: The Halfway Point
You have reached the midpoint of a typical 40-week pregnancy. Your baby is about 6.5 inches from crown to rump and weighs roughly 10.6 ounces. Swallowing increases as the digestive tract practices processing amniotic fluid, and meconium — the first stool — begins forming in the intestines.
The fundus (top of the uterus) reaches the navel at this stage. Mothers often report Braxton Hicks contractions, which are irregular, painless tightening sensations. These practice contractions are normal but should not occur more than four times per hour. Heartburn becomes more common as the expanding uterus pushes the stomach upward and pregnancy hormones relax the lower esophageal sphincter.
Week 21: Kicks Become Consistent
Your baby is 10.5 inches long and weighs about 14 ounces. Coordinated limb movements replace the jerky flutters of earlier weeks. You can feel distinct kicks, rolls, and hiccups. The bone marrow begins producing blood cells, a job previously handled by the liver and spleen.
Sleep disturbances affect many women by week 21. A combination of fetal movement, frequent urination, and difficulty finding comfortable positions contributes. Using pregnancy pillows between the knees and under the abdomen can reduce hip and lower back pressure. Mild ankle swelling (edema) is common in the evening but should resolve with elevation overnight.
Week 22: Rapid Brain Growth
Your baby measures 11 inches and weighs about 1 pound. Brain development accelerates dramatically, with neurons forming new connections at a rate of millions per minute. The cerebral cortex, responsible for higher cognitive function, begins folding to increase surface area.
The eyelids and eyebrows are fully formed. Taste buds develop, and the baby starts distinguishing sweet and bitter flavors from swallowed amniotic fluid. For mothers, increased vaginal discharge (leukorrhea) is normal as long as it remains clear or white and odorless. Green, yellow, or foul-smelling discharge warrants a call to the provider.
Week 23: Viability Milestone
Your baby is 11.4 inches long and weighs approximately 1.1 pounds. This week marks the beginning of the periviable period — the stage at which survival outside the uterus becomes theoretically possible, though outcomes remain highly variable. Lungs begin producing small amounts of surfactant, the substance that prevents air sacs from collapsing.
Rapid eye movements (REM) begin, suggesting the fetus experiences dream sleep. Blood vessels in the lungs expand to prepare for gas exchange after birth. Mothers may notice colostrum — early breast fluid — leaking from the nipples. This is normal and signals that the body is preparing for lactation.
Week 24: Lung Development Accelerates
The baby weighs about 1.3 pounds and measures 11.8 inches. The lungs develop bronchioles and primitive air sacs called alveoli. Surfactant production increases but remains insufficient for reliable independent breathing. The inner ear, which controls balance, is fully developed, allowing the baby to sense movement and position.
Mothers typically undergo glucose screening between 24 and 28 weeks to check for gestational diabetes. The test involves drinking a glucose solution and having blood drawn one hour later. Skin darkening may intensify, particularly around the areolas and on the face (melasma). Sunscreen with SPF 30 or higher helps prevent worsening of facial pigmentation.
Week 25: Sensory Maturation
Your baby weighs roughly 1.5 pounds and measures 13.6 inches. The hands are fully formed with fingernails, and the baby can grasp at the umbilical cord and other body parts. The nose, previously flat and plugged, opens for the first time, allowing the baby to practice breathing movements.
The skin, still thin and translucent, gains fat deposits that smooth the wrinkled appearance. The brain stem, which controls vital functions like breathing and heart rate, matures significantly. Mothers often report shortness of breath as the uterus pushes the diaphragm upward. This is typically benign but should be discussed with a provider if accompanied by chest pain or rapid heartbeat.
Week 26: Sleep-Wake Cycles Emerge
The baby weighs about 1.7 pounds and measures 14 inches. Sleep-wake cycles become more regular and can be detected via ultrasound. The eyes, which have been fused shut, may begin to open for brief periods. Brain wave patterns show distinct active and quiet sleep states.
The auditory system is fully functional. Studies indicate that fetuses respond to music and speech with changes in heart rate and movement (Kisilevsky et al., 2009). Mothers may experience lower back pain as the growing belly shifts the center of gravity forward. Pelvic tilts and prenatal yoga can help manage discomfort.
Week 27: End of the Second Trimester
You have completed the second trimester. Your baby weighs approximately 2 pounds and measures 14.4 inches. The lungs, while still immature, have enough surfactant to provide a chance of survival outside the womb with intensive medical support. The immune system receives antibodies from the mother for the first time, providing temporary protection after birth.
The brain continues rapid development, forming the complex connections necessary for learning and memory. Body fat accumulates at a faster rate. For mothers, the third trimester brings increased fatigue, more pronounced Braxton Hicks contractions, and preparation for delivery. Scheduling the Tdap vaccine (whooping cough booster) between 27 and 36 weeks is recommended to pass protective antibodies to the baby.
Common Second Trimester Symptoms
Understanding which symptoms are normal helps reduce anxiety. The most frequently reported second trimester experiences include:
- Round ligament pain: Sharp, brief aches in the lower abdomen, especially with sudden movement.
- Nasal congestion: Caused by increased blood flow to mucous membranes. Saline rinses are safe and effective.
- Leg cramps: Often occur at night and may relate to magnesium or calcium intake.
- Heartburn: Hormonal relaxation of the esophageal sphincter plus upward pressure from the uterus.
- Backache: Growing belly shifts posture; support belts and stretching help.
- Skin changes: Linea nigra, melasma, and stretch marks are hormonally driven and typically fade postpartum.
- Mild edema: Symmetrical ankle and foot swelling that resolves with elevation is normal.
Staying active with walking, swimming, or prenatal yoga supports circulation and reduces many of these symptoms. Hydration is critical — aim for at least 8–10 cups of water daily to support increased blood volume and amniotic fluid production.
Warning Signs to Watch For
While the second trimester is generally the most comfortable phase, certain symptoms require prompt medical attention:
- Vaginal bleeding: Any bleeding beyond light spotting should be evaluated immediately.
- Regular contractions: More than four contractions per hour before 37 weeks may signal preterm labor.
- Severe abdominal pain: Persistent, one-sided pain could indicate ovarian cysts or, rarely, ectopic complications.
- Sudden swelling: Rapid swelling in the hands, face, or one leg may indicate preeclampsia or deep vein thrombosis.
- Decreased fetal movement: After 24 weeks, a noticeable reduction in movement patterns warrants a call to your provider.
- Fever above 100.4°F (38°C): May indicate infection and requires evaluation.
- Leaking fluid: A continuous trickle or gush of fluid could indicate premature rupture of membranes.
Trust your instincts. If something feels wrong, contact your healthcare provider rather than waiting for the next scheduled appointment.
When to Call Your Pediatrician
Contact your provider if you experience vaginal bleeding, regular contractions (more than four per hour before 37 weeks), severe abdominal pain, sudden swelling, decreased fetal movement after 24 weeks, fever above 100.4°F (38°C), or leaking fluid. Trust your instincts — if something feels wrong, call your provider.
Conclusion
The second trimester spans 15 weeks of dramatic change for both mother and baby. From the first fluttering movements at week 16 to the emergence of sleep-wake cycles at week 26, the baby transforms from a lemon-sized fetus into a nearly 2-pound infant with functioning senses. For mothers, energy returns, nausea fades, and the pregnancy becomes physically visible. Staying informed through this week-by-week progression helps you anticipate changes, recognize normal symptoms, and identify warning signs early. Schedule regular prenatal visits, maintain a balanced diet rich in iron and calcium, and communicate openly with your provider about any concerns.
Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult your obstetrician or midwife with any concerns about your pregnancy.
Frequently Asked Questions
What week does the second trimester start and end?
The second trimester begins at week 13 and ends at week 27. It covers the middle third of a typical 40-week pregnancy.
When will I feel the baby move?
First-time mothers typically feel fetal movement between 18 and 22 weeks. Women who have been pregnant before may notice flutters as early as 13 to 16 weeks.
How much weight should I gain during the second trimester?
For women with a normal pre-pregnancy BMI, total recommended gain is 25–35 pounds. During the second trimester, this averages roughly 1 pound per week.
Is round ligament pain normal?
Yes, sharp brief pains in the lower abdomen or groin are common as ligaments stretch. Slow movements and gentle stretching help reduce discomfort.
Can I find out the baby’s sex during the second trimester?
Yes, the anatomy scan between 18 and 22 weeks can typically determine sex, though accuracy depends on fetal position and equipment.
When should I worry about decreased fetal movement?
After 24 weeks, any noticeable reduction in your baby’s normal movement pattern warrants a call to your provider. Do not wait until the next appointment.
What tests are done during the second trimester?
Common tests include the anatomy ultrasound (18–22 weeks), glucose screening for gestational diabetes (24–28 weeks), and blood work to check for anemia and Rh compatibility.
Is it safe to exercise during the second trimester?
Most low-impact exercises are safe and encouraged, including walking, swimming, and prenatal yoga. Avoid activities with fall risk or heavy lifting. Consult your provider before starting any new routine.
Why am I having trouble sleeping in the second trimester?
Frequent urination, fetal movement, heartburn, and difficulty finding comfortable positions all contribute. Left-side sleeping with pillows between the knees often helps.
What should I eat during the second trimester?
Focus on iron-rich foods, calcium, omega-3 fatty acids, and adequate protein. Avoid unpasteurized dairy, raw fish, deli meats unless heated, and excessive caffeine (under 200 mg per day is generally considered acceptable).
This article is for general informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your pediatrician or obstetrician about your individual health needs. If you believe you have a medical emergency, call your local emergency number immediately.
