First trimester · 4 weeks pregnant
Week 4: a missed period and a positive test
The embryo is settling in and hormone levels are rising fast.
Implantation finishes and the embryo starts forming the structures that will support it: the amniotic sac, the yolk sac and the beginnings of the placenta. A home test can turn positive from about now.
Size and weight
About the size of
a poppy seed
Length
0.1–1 mm
Weight
—
Averages from standard growth charts. Measured crown to rump until week 20 and head to heel afterwards.
How your baby is developing
- The blastocyst is now fully implanted and begins releasing hCG into your blood.
- The embryo forms two layers, then three: the beginnings of skin and nerves, organs, and muscle and bone.
- The amniotic sac and yolk sac form; the yolk sac feeds the embryo until the placenta takes over.
How you may feel
- A missed period
- Tender, fuller breasts and darker nipples
- Tiredness
- Light spotting or mild cramping
- A metallic taste or heightened smell
A home pregnancy test is reliable from the first day of your missed period. If it is negative but your period does not come, repeat it in two or three days.
What's next
Tests and appointments
- Take a home pregnancy test.
- If positive, register at the prenatal office (women's consultation) of your district polyclinic. The national protocol recommends the first visit by 10 weeks.
What to do this week
- Continue folic acid (1 mg) and potassium iodide (200 µg) daily.
- Stop alcohol completely and stop smoking; ask for support if quitting is hard.
- Avoid raw or undercooked meat, unpasteurised dairy and soft cheeses, raw eggs, liver and high-mercury fish.
- Let someone else change the cat litter, or wear gloves and wash your hands well.
Call your doctor or maternity hospital if
- Sharp pain on one side with bleeding, shoulder-tip pain or fainting can signal an ectopic pregnancy: seek urgent care.
- Bleeding that is heavier than light spotting, or bleeding with cramps
- Severe pain on one side of your lower abdomen, or pain with dizziness or fainting
- Vomiting so often that you cannot keep fluids down for 24 hours
- A temperature of 38 °C or higher, or pain when you pass urine
If something feels wrong, trust yourself and call 103 or your maternity hospital. You will never be wasting anyone's time.
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