Fetal development: what happens week by week and what you can do

Pregnancy moves fast and every week matters. Knowing the main milestones and simple actions you can take gives you control and helps you spot problems early. Below is a straightforward guide to how the embryo/fetus grows and what to watch for in each phase.

Key milestones by trimester

First trimester (weeks 1–12): cells form organs. The embryonic phase (weeks 3–8) is when organs start developing — that’s why folic acid before and during early pregnancy is crucial. By about 6 weeks you can often see a heartbeat on ultrasound. At 8–12 weeks limbs, fingers, and facial features take shape. Around week 12 most major structures are formed and the risk for major structural problems drops.

Second trimester (weeks 13–27): growth and movement. The fetus grows quickly. By 14–20 weeks you might feel little flutters (quickening) if this is not your first pregnancy. Anatomy scans usually happen at 18–22 weeks to check organs and placenta. At around 20 weeks the fetus is roughly 25 cm head-to-heel and weighs a few hundred grams — big enough for clearer movement and stronger heart sounds.

Third trimester (weeks 28–40): weight gain and final maturation. The lungs and brain mature most in these weeks. Fetal kicks become regular; doctors track them to monitor well‑being. Full term is 39–40 weeks. Babies born after 37 weeks usually do well without extra support, but earlier births can need extra care.

Practical tips for a healthier pregnancy

Take a daily prenatal vitamin with 400–800 mcg folic acid early on. That’s the single best step to reduce neural tube defects. Eat balanced meals with iron, calcium, protein, and avoid raw or unpasteurized foods. Limit high‑mercury fish (shark, swordfish), and skip alcohol and recreational drugs entirely.

Ask your provider about vaccines — flu and Tdap are commonly recommended in pregnancy — and always check before taking any medicine. Many prescription drugs are safe, some aren’t. If you’re on chronic medication, talk to your clinician before changing anything.

Keep track of fetal movement after 28 weeks. A simple kick count (two hours with at least 10 movements) helps you notice changes. Call your provider right away for heavy bleeding, severe belly pain, sudden swelling, high fever, or a big drop in baby’s movements.

Stay active with pregnancy‑safe exercise, manage stress, and get regular prenatal visits. Ultrasounds and standard screening tests (first‑trimester screening, anatomy scan, glucose test) give useful checkpoints — ask questions during each visit so you understand results and next steps.

Fetal development can feel overwhelming, but small, consistent steps make a big difference. Follow your care plan, ask questions, and report concerns early — that’s the simplest way to protect you and your baby through pregnancy.