1st trimester · Week 4
4 weeks pregnant
Implantation and the first positive tests
Around now a blastocyst may nestle into the endometrium and start making hCG. A missed period and a positive home test often appear this week. You are still in the earliest embryonic stage.
How big is your baby at 4 weeks?
- Length
- 0.2 cm
- Size
- Seed
On average about 0.2 cm long. Size comparison: Seed.
These are averages; every baby grows at their own pace.
How is your baby developing at 4 weeks?
After implantation, a tiny embryonic disc and yolk sac begin to organize. Three germ layers will later become organs, but no formed baby exists yet. Average length is only a couple of millimeters; weight is essentially unmeasurable.
What changes in your body at 4 weeks?
Tender breasts, fatigue, or fuller-than-usual smell can appear, or you may feel unchanged. Light implantation spotting can happen. Cramping similar to a period is common; heavy bleeding or clot-like tissue should be assessed.
Tips for this week
Confirm a positive test with your clinician when you can, especially if you have pain, bleeding, or a history of ectopic pregnancy. Continue folate. Pause alcohol, and ask before taking painkillers or herbal products. Most early pregnancies are intrauterine — but only an exam or ultrasound can locate it.
