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Why does pregnancy start at week 4 instead of week 1? This can be a little confusing, so let’s break it down.
Forty weeks of pregnancy actually begin with the first day of the last period you had. During weeks 1 and 2, you haven’t conceived yet. Your uterine lining is thickening to be ready to receive a fertilized egg. Conception usually happens between weeks 2 and 3.
During week 3, there’s a lot going on inside your body. Even though a pregnancy test won’t be positive yet, the first stages of embryo development are already happening.
The fertilized egg travels through the fallopian tube toward your uterus. The cells in the fertilized egg continually divide. The egg (now called a blastocyst) grows quickly.
Near the end of week 3, the blastocyst reaches your uterus where it:
- Attaches to the uterine lining (called implantation)
- Becomes an embryo
You might notice some light bleeding (spotting) as the egg implants in the uterine lining.
At the end of week 3, the embryo is tiny, about the size of the head of a pin, and continues to grow quickly.
You’re 4 weeks pregnant at the time of your missed period, and if you take a pregnancy test, you’ll probably get a positive result. During week 4, the embryo is forming the heart, brain, and lungs and is about the size of a sesame seed.
The cells of the embryo divide rapidly. The amniotic sac forms around your baby and fills with fluid which will surround and cushion your baby throughout pregnancy.
The embryo receives blood and nutrients from a yolk sac that develops during implantation, until the placenta fully forms and takes over the job.