First trimester
4 weeks pregnant
Implantation completes and the pregnancy starts producing hCG, the hormone that home tests detect. This is the week many people get a positive result, often right around the time a period is due.
Size of
poppy seed
Length
0.5 mm
Weight
< 0.001 g
Your baby at 4 weeks
The embryo is a flat disc of three layers, each of which will become a different part of the body. The ectoderm forms the nervous system and skin; the mesoderm forms bone, muscle, and the heart; the endoderm forms the gut and lungs.
The amniotic sac and yolk sac form around the embryo. The yolk sac supplies nutrients until the placenta takes over, which happens gradually over the first trimester.
The placenta begins developing in earnest, burrowing into the uterine lining to establish the blood supply that will sustain the pregnancy.
How you may be feeling
A missed period is usually the first sign. Home pregnancy tests detect hCG in urine and are generally accurate from around the day a period is due, though testing a few days later reduces the chance of a false negative.
Some people feel early symptoms already — sore breasts, fatigue, a metallic taste, heightened sense of smell. Others feel nothing at all for several more weeks. Both are entirely normal and neither predicts how the pregnancy will go.
Common symptoms around week 4
- A missed period, usually the first noticeable sign
- Tender or swollen breasts, often one of the earliest changes
- Fatigue that feels different in kind from ordinary tiredness
- Mild cramping, which many people find alarming but is common
Appointments and tests around now
Once you have a positive test, contact a midwife or doctor to arrange booking. In many systems the first appointment is offered somewhere between weeks 8 and 12, and getting into the system early gives you more options for screening.
What is normal, and what varies
A faint line on a test is still a positive. hCG rises quickly in early pregnancy, so a line that was faint one day is typically darker two days later.
Worth doing this week
Test with first-morning urine
hCG is most concentrated then, which makes an early positive easier to detect and a faint line easier to read.
Book in now, even if the appointment is weeks away
Waiting lists exist. Making contact early does not mean being seen early, but it protects your place.
Do not panic about mild cramping
The uterus is changing rapidly and mild cramps are very common. Pain that is severe, one-sided, or paired with bleeding is the version worth calling about.
Looking ahead
Next week the heart begins to form and beat, usually before you have had a single appointment.
Nearby weeks
Not sure which week you are in? The week tracker works it out from the first day of your last period, and the due date calculator gives you an estimated due date.