Second Trimester: Your Baby's Development

The second trimester (weeks 13-26) is the period when your baby rapidly grows and matures. At week 13, the tiny being is approximately 7 cm and 25 grams; by the end of week 26, they reach 35 cm and 760 grams — what a tremendous change!

Weeks 13-16: Facial expressions begin to form — frowning and lip pouting can be seen on ultrasound. Bones harden, joints move. Sex can be determined during this period (weeks 14-16) on ultrasound. Fine hair called lanugo begins to cover the body and kidneys are actively working.

Weeks 17-22: Between weeks 16-25 (usually after week 18 in a first pregnancy), you may feel the first baby movements (quickening) — those gentle tremors like butterfly wings. Hearing is developing too; they can hear your voice, heartbeat, and music. The detailed ultrasound at week 20 evaluates all organs, growth, and placenta position.

Weeks 23-26: Lungs begin producing surfactant. Eyelids open and detect light changes. As the brain rapidly develops, the sleep-wake cycle also begins to form. Week 24 is considered the "viability threshold" — from this week onward, babies born early can survive with intensive care support.