Baby development at week 4
By week 4, the amniotic sac is forming and the embryo is roughly poppy-seed scale. Cells are organizing into structures that will become baby and placenta.
Average size estimates for week 4 are about 0.1 cm and too early to weigh meaningfully — roughly poppy seed stage. Your ultrasound report and clinician’s dating override any website average.
What you might feel
Breast tenderness, fatigue, or a missed period may appear. Nausea is possible but not universal.
You are in the first trimester. Two pregnancies at the same week can feel completely different — treat these notes as orientation, not a checklist you must match.
What to track this week
- Symptoms, mood, sleep, and energy in your daily log
- Water intake toward a simple daily goal
- Appointment dates and questions for your care provider
A calm next step in Preglo
Confirm with a test, book prenatal care, and set your due date estimate in Preglo so week tracking stays calm and consistent.
Also useful: due date calculator · all weeks · Preglo features
Nearby weeks: Week 2 · Week 3 · Week 5 · Week 6
Educational averages only — not medical advice. Contact your clinician for symptoms that worry you, bleeding, severe pain, or reduced movements.