First Trimester Baby Development: What Happens Week by Week
The fastest construction project in nature
The first trimester runs from week 1 (counted from your last period) to the end of week 12. Almost everything structural happens here: the neural tube that becomes the brain and spine, the heart, the limbs, the face, and every major organ system is laid down in this window. In this video I take you week by week through what forms when — and what you might see at an early ultrasound.
Key milestones
- Weeks 3–4: Fertilisation, implantation, and the missed period. The neural tube begins forming.
- Weeks 5–6: The heart starts beating — often visible on an early scan around week 6. Arm and leg buds appear.
- Weeks 7–8: Rapid brain growth; fingers begin; the baby starts making tiny movements.
- Weeks 9–10: All essential organs have begun. Elbows bend, toes form, and the embryo officially becomes a fetus.
- Weeks 11–12: Fingernails, teeth buds, and reflexes. By week 12 your baby is recognisably a tiny human, around 5–6 cm long.
Why these weeks deserve extra care
Because organs form in this window, the first trimester is when your baby is most sensitive to alcohol, certain medicines, and infections. Folic acid before conception and through week 12 protects the spine and brain. If you find out you're pregnant later than you planned, don't panic — just start now, tell your provider, and go forward from there.
Common questions
When can the heartbeat be seen?
A heartbeat is usually visible on a transvaginal ultrasound from around week 6, and heard on a Doppler later, commonly around weeks 10–12. If an early scan shows no heartbeat yet, it often simply means you're earlier than you thought — you'll usually be re-scanned in a week or two.
How big is my baby at 12 weeks?
About the size of a lime — roughly 5–6 cm from crown to rump and about 14 grams.
Why am I so tired if the baby is so small?
You're building a placenta, increasing your blood volume, and running high progesterone — all while the baby does its fastest growing. First-trimester exhaustion is your body working exactly as it should.
Your Baby's First 40 Weeks in the Womb
Now widen the lens: the full forty-week journey, week by week.