1. 4 WEEK FETUS

At 4 weeks pregnant, the developing baby is formally called an embryo (not a fetus) and is about the size of a poppy seed, measuring roughly

. It has implanted in the uterus and is forming layers that will become organs, along with the amniotic sac and placenta. 

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Key 4-Week Development Details

• Structure: The embryo is in a stage of rapid cell division and has implanted into the uterine lining.
• Layers: Three cell layers are forming (ectoderm, mesoderm, endoderm) that will eventually develop into the nervous system, skeleton, muscles, and organs.
• Support Systems: The amniotic sac (protects), yolk sac (produces blood), and primitive placenta are forming.

• Heart: Although not beating yet, the initial structure for the heart has begun to form, with a heartbeat possible by weeks 5 or 6. 

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Symptoms and Physical Changes

• Implantation: Some women experience light bleeding or cramping.
• Hormones: Human Chorionic Gonadotropin (hCG) is now being produced, resulting in a positive home pregnancy test.
• Symptoms: While many feel no symptoms yet, some may begin experiencing early fatigue or breast tenderness.