First Trimester: What's Happening with Your Baby?

The first trimester (weeks 1-12) is the period when your baby develops most rapidly — truly a miraculous time. By week 4, the fertilized egg implants in the uterine wall and the embryo begins to form. By week 5, that tiny heart begins to beat — approximately 100 beats per minute. This little heart will work tirelessly for you throughout the pregnancy.

Between weeks 6-8, the brain, spinal cord, eyes, ears, and hand-foot buds begin to take shape. At week 8 your baby is about 1.5 cm long; by the end of pregnancy week 10 (week 8 counted from conception), the embryo is called a "fetus". Since all basic organs form during this period, the first trimester is the most critical period of development.

Between weeks 9-10, fingers separate and nails begin to appear. The baby moves but you can't feel it yet. By week 11, bone marrow begins producing blood cells and kidneys produce urine. Although sex is genetically determined at the moment of fertilization, it typically becomes visible on ultrasound around weeks 14-16, and is assessed most reliably at the detailed anomaly scan at weeks 18-22.

By the end of week 12, your baby is approximately 6 cm long and weighs 14 grams. Reflexes have begun to develop — they can make swallowing and sucking movements. After this week, the risk of miscarriage significantly decreases. First trimester screening tests (double test, nuchal translucency measurement) are performed between weeks 11-14.