Week 4 of Pregnancy

Length

0.1 cm

Crown-rump (CRL)

Weight

Not yet measurable

Fetal development

• The embryo fully implants into the uterine lining.
• Placenta and amniotic sac begin to form.
• The embryo is the size of a poppy seed; three germ layers (ectoderm, mesoderm, endoderm) begin to differentiate.

Maternal changes

• hCG rises quickly, often making home pregnancy tests positive.
• Mild fatigue, breast tenderness, or light nausea may appear.

Recommended checkups

• A positive urine test means time to book the first OB appointment, typically scheduled around weeks 7–8.

Related calculators

• Due date Calculate pregnancy week and due date from your last period
• Pregnancy test timing Find when to test from your last intercourse or ovulation
• βhCG doubling time Calculate hCG doubling time and daily rise from two early pregnancy blood tests
• Cycle Calculate next period, ovulation, and fertile window

Related topics

• Implantation Bleeding Light early-pregnancy spotting — timing, color, vs. a period
• Early Pregnancy Symptoms The 13 most common signs in the first 1–4 weeks
• Pregnancy Foods What to eat, what to avoid, and what to limit

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