Illustration of prenatal development at week 4 — Microscopic view, greatly enlarged.

First trimester, week 4

4 weeks pregnant: Missed period, positive test

Size of a: Poppy seed
Length: ≈ 2 mm embryonic disc
Weight: —
Heart rate: — not yet
Due date: 252 days away 10% of the way

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Checked against the sources listed on this page. Not reviewed by a clinician. Last updated 5 November 2026. How we check.

At 4 weeks pregnant, the embryo is a tiny disc of cells about 2 mm long, roughly the size of a poppy seed, and it has just implanted in the womb.

Your baby this week

Now safely implanted, the embryo is a flat disc of cells, with the amniotic cavity on one side and the yolk sac on the other.
Cells begin to organise into layers that will become every organ: skin and nerves, muscle, bone and heart, and the gut and lungs.
Early placenta tissue (the chorionic villi) is tapping into your blood supply.

Your body this week

For a 28-day cycle, your period is due at the start of this week. A home test is usually positive from the day it is missed.
hCG roughly doubles every 2–3 days in early pregnancy. It can cause tiredness, sore breasts and needing to pee more often.
Some people feel nothing at all yet. That is completely normal.

Typical weight gain so far: 0.1–0.5 kg
Your uterus: About the size of a pear (unchanged to the eye)

Weight gain range is for a pre-pregnancy BMI of 18.5–24.9 (IOM guideline). Your provider may advise differently.

Can you trust a positive test at 4 weeks?

A positive result is almost certainly correct. Most home tests are reliable from the first day of a missed period, which is around now, and some very sensitive ones can work a little earlier.

A negative result is less reliable, because it may simply be too early. If you still think you’re pregnant, wait a few days and test again. See a doctor if a second test is negative and your period still hasn’t come.

What to do first at 4 weeks

Contact a midwife or doctor to start your care. You usually won’t be seen straight away, as the first full appointment in the UK is usually between 8 and 12 weeks. See your doctor sooner if you take regular medicine or have a long-term condition.

Start taking 400 micrograms of folic acid a day if you haven’t already, and keep going until at least week 12. A higher dose is prescribed for some people, so ask if you’re unsure. Vitamin D is also advised. Avoid supplements containing vitamin A (retinol).

If you drank, used cannabis or took medicine before you knew

This happens a lot. The NHS advises not drinking at all once you know you’re pregnant, but says the risk from drinking before you knew is likely to be low and you shouldn’t worry unnecessarily. Just avoid alcohol from now on, and mention it to your midwife if it’s on your mind.

Don’t stop a prescribed medicine on your own, because that could be harmful too. Check with a pharmacist, midwife or doctor before taking any medicine, including painkillers. See what’s known about common medicines.

If you used cannabis once before you knew, the NHS says to try not to worry, because it’s unlikely to have affected your baby. The CDC recommends not using it in pregnancy, so avoid it from now on and tell your midwife or doctor.

What you might feel at 4 weeks

You might notice sore breasts, tiredness, nausea, a metallic taste, bloating or needing to pee more often. Light spotting and mild cramps can also happen around now. You might also feel nothing at all, which is common.

Get urgent care for strong pain on one side of your belly, pain at the tip of your shoulder, or feeling faint. These can be signs of an ectopic pregnancy, and symptoms usually develop between weeks 4 and 12.

Scans, tests and checkups

This week

Home pregnancy test Self-care
Detects hCG in urine. Most reliable from the first day of a missed period.
From week 6

First prenatal visit & bloodwork Checkup
Health history, blood pressure, urine, blood type and Rh factor, blood count, and infection screening (hepatitis B, HIV, syphilis, rubella immunity). The UK aims to see you by 10 weeks.
From week 6

Early / dating ultrasound Ultrasound
Confirms the pregnancy is in the uterus, sees the heartbeat, counts babies and dates the pregnancy. Often part of the first visit or combined with the 12-week scan.

See the full scan and test timeline

Real size, next to an adult hand

Adult hand 18.5 cmbaby 2 mmThe ring marks a stage too small to see at true scale.

Growth curve

Typical weight by week: WHO median from week 20, approximate before that. Healthy babies vary widely.

When to call your midwife or doctor at 4 weeks

Call your midwife, doctor or maternity unit straight away, day or night, if you have any of these:

Vaginal bleeding, with or without cramps.
Strong pain low on one side of your belly, pain at the tip of your shoulder, or feeling faint. These can be signs of an ectopic pregnancy, which needs urgent care.
Being sick so often that you can’t keep fluids down for a day.
Fever of 38 °C (100.4 °F) or higher, or pain or burning when you pee.
Pain or swelling in one leg, chest pain, or sudden shortness of breath. These can be signs of a blood clot.
Feeling very low, hopeless or anxious most days. If you feel you might harm yourself, get emergency help.

Based on NHS and ACOG guidance. This list isn’t complete. If something feels wrong, call anyway. In an emergency, call your local emergency number.

Tip for this week

Contact a midwife or doctor to arrange your first appointment. In the UK it is usually between 8 and 12 weeks. In the US the first visit is often around 8–10 weeks.

Myth or fact?

Does having no pregnancy symptoms mean something is wrong?

Myth

No. Many people have healthy pregnancies with few or no symptoms, especially early on.

Read the evidence

Common questions at this stage

All common questions

Questions people ask about week 4

How accurate is a pregnancy test at 4 weeks?

Many home tests claim over 99% accuracy from the day your period is due. In practice, some pregnancies aren’t detected until a few days later. If the test is negative and your period still hasn’t come, test again in 2–3 days.

How big is the baby at 4 weeks?

About 2 mm, roughly the size of a poppy seed. It is a flat disc of cells rather than a recognisable shape.

References

Advice differs between countries, so follow your own care team’s plan. Weights from week 20 are the WHO median, and earlier weights are approximate. How we check this.