4 Weeks Pregnant: Ultrasound and Early Pregnancy – What to Expect

At 4 weeks gestation (2 weeks post-conception), an ultrasound cannot visualize an embryo. The developing blastocyst is microscopic (~0.1-0.2 mm)—a cluster of cells implanting in the uterine lining.

What Ultrasound Can Detect and When

• 4-5 weeks: Thickened endometrium; possibly a tiny gestational sac (2-3 mm) via transvaginal ultrasound.
• 5.5-6 weeks: Yolk sac visible inside gestational sac.
• 6-7 weeks: Fetal pole with cardiac activity detectable.
• 7-8 weeks: Clear embryonic structure, heartbeat ~120-160 bpm.

Early Pregnancy Confirmation

• Home pregnancy tests: Detect hCG in urine ~10-14 days post-conception (around missed period).
• Quantitative blood hCG: More sensitive; tracks doubling time (every 48-72 hours in viable pregnancy).
• Progesterone levels: Support viability assessment.

What to Expect at 4 Weeks

• Missed period (if regular cycles).
• Mild cramping, breast tenderness, fatigue, nausea may begin.
• Start prenatal vitamin with folic acid (400-800 mcg) immediately.
• Schedule first prenatal visit for 8-10 weeks.

An early ultrasound before 6 weeks is rarely indicated unless evaluating for ectopic pregnancy, bleeding, or uncertain dates.