First trimester
4 weeks pregnant: what is happening this week
Week 4 of 42worked example
2 mm
- Measured
- head to bottomabout the size of a poppy seed
- This week runs
- Thu, Jan 22, 2026 to Wed, Jan 28, 2026
- Days to the estimated due date
- 259
- Term window (37-42 weeks)
- Thu, Sep 17, 2026 to Thu, Oct 22, 2026
The NHS publishes this figure for week 4, about the size of a poppy seed. It is an average from patient guidance rather than a measurement of any particular pregnancy, and the measurement basis changes at week 20 - see the note below the dates.
Two millimetres. That is the NHS figure for this week, and it is the first week the guide gives a size at all - roughly a poppy seed, which is small enough that the comparison does more work than the number. Implantation happens around now, and with it the first hormone a test can find, which is why week four is when most positives appear. It is also the week the supporting apparatus starts: an amniotic sac with fluid in it, a yolk sac doing the feeding for the moment, and an outer layer of cells that will become the placenta. The embryo itself is a fraction of what is going on. A positive test this week is doing something oddly specific - it is telling you about a hormone level, not about how far along you are. The count is already at four weeks because of where the count started, not because four weeks of development have happened. If there was any spotting a few days ago, that timing is common enough to have a name, and it is not the same thing as a period even when it arrives when one was due. The date in the calculator should still be the last period, not the spotting.
Put your own dates on week 4
- Week 1 began
- Thu, Jan 1, 2026
- Week 4 begins
- Thu, Jan 22, 2026
- Week 4 ends
- Wed, Jan 28, 2026
- Estimated due date
- Thu, Oct 8, 2026
Dates run on ACOG's 280-day convention, the same arithmetic as the due date calculator. Share this week with your dates: /pregnancy/week-4?s=d:2026-10-08
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What the NHS describes this week
- The NHS puts the embryo at around 2 mm this week - the first week its guide gives a measurement.
- The amniotic sac has formed around the embryo and is filled with fluid that cushions it.
- A yolk sac is doing the feeding, while the outer layer of cells begins to form what becomes the placenta.
What happens around now
A missed period is the usual prompt for a test. NHS guidance is to contact a GP or midwife once a pregnancy is confirmed, so that the booking appointment can be arranged for somewhere between weeks 8 and 12.
On the measurement. The NHS publishes a length for each week from 4 to 40. Up to week 19 it is measured head to bottom (crown to rump, the measurement a scan takes while the legs are curled). From week 20 it is measured head to heel, which is why the figure jumps from 15.3 cm at week 19 to 25.6 cm at week 20 - the baby did not grow 10 cm in a week, the ruler changed. The NHS states the change on its week 20 page.
Where week 4 comes from
- NHS: week 4 of pregnancy
- NHS: your antenatal appointments — the booking appointment is offered between 8 and 12 weeks
- NHS, Week-by-week guide to pregnancy — the guide these weeks are built from
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