Week 4 is usually the week you find out. Your period is due, hCG has climbed high enough for a home test to detect it, and what was invisible last week becomes a fact.
The embryo is around 1 mm long and has just done something remarkable: divided itself into the three tissue layers that every organ in the body will be built from.
The short answer: a home test is normally reliable now, especially with first-morning urine, and a faint line counts. The embryo is about 1 mm and is forming its three germ layers — ectoderm, mesoderm and endoderm — which is the start of organ development. Book your first antenatal appointment this week; it usually takes place between 8 and 12 weeks and the first screening window closes at 14.
Your baby this week
This week the embryo undergoes gastrulation — the cells reorganise into three distinct layers, and from this point every structure in the body traces back to one of them:
- Ectoderm — skin, hair, tooth enamel, and the entire nervous system including the brain and spinal cord
- Mesoderm — the skeleton, muscles, heart, blood vessels, kidneys and reproductive organs
- Endoderm — the lining of the gut, plus the liver, pancreas, thyroid and lungs
This is the beginning of organogenesis, the period when organs are physically built rather than merely growing. It runs from now to about week 10 and is the window in which the developing embryo is most sensitive to alcohol, certain medications and infections — which is exactly why medication safety is worth checking now rather than at your first appointment.
Alongside the embryo, the amniotic sac and yolk sac are forming, and the trophoblast is building what will become the placenta.
Your body this week
The trophoblast is now secreting human chorionic gonadotropin (hCG) in earnest. Its job is to keep the corpus luteum alive so it continues producing progesterone — without that signal, the lining would break down and your period would arrive. hCG roughly doubles every 48 to 72 hours in early pregnancy, which is why a test that was negative three days ago can be clearly positive now.
hCG is also responsible for most of what you feel. The sharp shift in blood chemistry drives the tiredness, the tender breasts, the altered sense of taste and smell, and eventually the nausea. Mild cramping is common and, on its own, not a warning sign — the uterus is changing shape and the ligaments supporting it are being stretched.
If you feel nothing at all this week, that is not a bad sign. Symptom intensity varies enormously between women and between pregnancies in the same woman, and it does not predict how a pregnancy will go.
Tests and appointments
Tests and scans around now: A home pregnancy test becomes reliable around now, once hCG is high enough to detect.
Exact timing varies by country and by your own care plan — your doctor or midwife will confirm the schedule that applies to you.
Once a test is positive, the practical next step is booking your first antenatal appointment. That visit typically covers blood group and rhesus status, a full blood count, and screening for HIV, hepatitis B, syphilis and immunity to rubella, along with your medical and family history. Booking early is not fussiness: the combined first-trimester screening window runs from 11 to 14 weeks, and appointments have to be arranged before it.
✅ Worth doing this week
- Book the first antenatal appointment — the screening window closes at 14 weeks and slots fill up
- Review every medication and supplement with a doctor or pharmacist before stopping anything yourself; some conditions are more dangerous untreated
- Continue folic acid — the neural tube closes over the next two weeks and this is the window it matters in
- Cut out alcohol completely from now, as organogenesis is under way
- Note the date of the positive test and the first day of your last period — you will be asked for both
When to call your doctor
Call the same day if: you have heavy bleeding, severe or one-sided abdominal pain, shoulder-tip pain, dizziness or fainting. A positive test with severe one-sided pain needs urgent assessment for ectopic pregnancy, which cannot be diagnosed from symptoms alone and is dangerous if missed.
Light spotting in early pregnancy is common and often comes to nothing, but it should always be mentioned to your doctor rather than assumed to be harmless — particularly if you have a rhesus-negative blood group, where bleeding can affect management.
FAQ — 4 weeks pregnant
Will a test be positive at 4 weeks?
Usually yes. This is the week your period is due, and hCG has normally risen far enough to be picked up in urine. Test with the first urine of the morning, when hCG is most concentrated. A faint line is still a positive line. A negative test with a missed period is worth repeating in two or three days, because implantation may simply have happened later than average.
What symptoms are normal at 4 weeks?
Sore or tingling breasts, tiredness that feels disproportionate, mild cramping, a metallic taste, sensitivity to smells, and needing to pass urine more often. Some women feel nothing at all, and that is equally normal — symptom intensity is not a measure of how well a pregnancy is going. Nausea usually starts a little later, around week 5 or 6.
How likely is a miscarriage at 4 weeks?
Roughly 10–20% of confirmed pregnancies end in miscarriage, and the great majority of those happen in the first trimester. The most common cause by far is a chromosomal error present from the moment of fertilisation — nothing the mother did, ate, lifted or worried about. That is not reassurance for its own sake; it is the reason no amount of resting changes the outcome at this stage.
When should I contact a doctor after a positive test?
Book your first antenatal appointment now — it is usually held between 8 and 12 weeks, and booking early matters because the first-trimester screening window closes at 14 weeks. Contact a doctor sooner if you take regular medication, have a long-term condition, have had an ectopic pregnancy or recurrent miscarriage before, or if you have pain or bleeding.
Week 4 at a glance
Week 4 is the week a home test normally works, because hCG has doubled every two to three days since implantation. The embryo is about 1 mm and has just formed the three germ layers that every organ will be built from, which marks the start of organogenesis — the period when the embryo is most vulnerable to alcohol, certain drugs and infections. Tiredness, tender breasts and a metallic taste are common; feeling nothing is equally normal and predicts nothing. The practical job this week is booking the first antenatal appointment, because the first-trimester screening window closes at 14 weeks.
Inside Baby Novum: enter the first day of your last period and the app produces your due date, your current week, and the windows for every screening test — including the 11–14 week one you need to book around. Appointment reminders and the symptom journal start from here.