4 WEEKS PREGNANT
A tiny cluster of cells embeds into the uterine lining
Baby this week: about 0.1 cm — smaller than a grain of rice · under 1 g
≈ 0.1 cm — about the size of a poppy seed
Illustration enlarged for clarity — the amber marker shows true relative size
Your baby this week
- The fertilized egg, now called a blastocyst, burrows into the uterine wall
- Cells are already sorting into two groups — one that will become the embryo, one that will become the placenta
- The placenta and early umbilical cord begin forming
- Levels of the pregnancy hormone hCG start climbing, which is what a home test detects
Your body this week
- A missed period is often the first sign something has changed
- Some notice light spotting or cramping as implantation happens, though many feel nothing at all
This week's tip: If a home test comes back positive, book an appointment with a doctor or midwife to confirm the pregnancy and talk through early prenatal care.
Want the bigger picture — organ development, nutrition, scans and warning signs for this stage? See month 1: The invisible beginning.
Sizes and milestones are typical ranges — every pregnancy runs on its own clock. Educational content; your prenatal care team's guidance always comes first.