Second Trimester Guide — Pregnancy Weeks 13 to 27

Weeks 13 to 16 — Energy Return and Symptom Relief

The second trimester begins at week 13 and is widely regarded as the most comfortable phase of pregnancy. By weeks 13 to 16, the placenta has taken over progesterone production from the corpus luteum, which often leads to a dramatic reduction in nausea and a significant energy rebound. Morning sickness, which affected an estimated 70% to 80% of pregnant individuals, typically resolves between weeks 14 and 16. Breast tenderness diminishes, fatigue lifts, and many people describe a renewed sense of wellbeing. The fetus grows rapidly during this window, reaching about 11.6 cm (4.6 inches) and 70 grams (2.5 ounces) by week 16. Swallowing and kicking movements are visible on ultrasound but are not yet felt by the mother.

Weeks 17 to 20 — Quickening and the Anatomy Scan

Between weeks 17 and 20, most pregnant individuals experience quickening — the first perception of fetal movement. First-time mothers typically feel quickening later, around weeks 18 to 20, because they have less experience distinguishing the sensation from intestinal gas. The detailed anatomy ultrasound is typically scheduled between weeks 18 and 22 and provides a comprehensive examination of fetal anatomy including the brain, heart, spine, kidneys, stomach, face, and limbs. This scan can detect approximately 70% of major structural abnormalities and is also used to assess amniotic fluid volume and placental location. If placenta previa is detected, the provider will recommend additional monitoring throughout the remainder of the pregnancy.

Weeks 21 to 24 — Viability and Hearing Development

By week 21, the fetus can hear sounds from outside the womb, and studies have shown that infants can recognize voices and music heard during pregnancy. The fetus begins developing REM sleep patterns and may dream. Vernix caseosa — a protective waxy coating — covers the fetal skin, and lanugo (fine hair) covers the body to help retain vernix. By week 24, the fetus is considered potentially viable with intensive NICU support, and survival rates at 24 weeks exceed 60% at Level III and Level IV neonatal intensive care units. The lungs are beginning to produce surfactant but are not yet mature enough for independent breathing. Weight gain in this window is typically around 230 grams (0.5 lb) per week for individuals with a normal pre-pregnancy BMI.

Weeks 25 to 27 — Lung Maturation and Third-Trimester Approach

Weeks 25 to 27 bring rapid lung development as surfactant production increases significantly. The brain grows rapidly and develops gyri and sulci, the characteristic folds that increase cortical surface area. By week 26, the fetus weighs about 760 grams (1.7 lb) and is about 34.4 cm (13.6 inches) long. Eyes begin opening, and the fetus can distinguish between light and dark. The World Health Organization (WHO) recommends that all pregnant individuals receive routine iron supplementation during the second and third trimesters, as iron demands increase by approximately 4 mg per day to support expanded blood volume and fetal iron stores. The end of week 27 marks the transition into the third trimester, when kick count monitoring becomes a standard self-assessment practice.

Key Prenatal Tests in the Second Trimester

Several important prenatal tests occur during the second trimester. The quad screen, performed between weeks 15 and 22, measures four maternal serum markers — AFP, hCG, estriol, and inhibin A — to screen for neural tube defects and chromosomal abnormalities. The glucose screening test, recommended between weeks 24 and 28, involves drinking a 50 g glucose solution and measuring blood sugar one hour later; a result above 140 mg/dL typically requires a follow-up 3-hour oral glucose tolerance test to diagnose or rule out gestational diabetes. ACOG recommends universal screening for group B streptococcus (GBS) between weeks 35 and 37, but second-trimester visits also include blood pressure monitoring, weight tracking, fundal height measurement (which begins tracking around week 20), and discussion of any emerging concerns. An additional Rhogam injection is given at week 28 for Rh-negative mothers to prevent hemolytic disease of the newborn.