Week 4 of Pregnancy
Implantation completes and hCG begins to rise. The earliest urine pregnancy tests may turn positive this week.
Fetal size
Poppy seed
- 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.
Jump to week
Textbook averages. Individual variation is wide and this is not medical advice — confirm with your OB.