Pregnancy Week 4: Implantation, Your First Positive Test and Early Symptoms

Quick answer: Week 4 is when implantation happens: the blastocyst burrows into your uterine lining and starts releasing hCG, so a home pregnancy test usually turns positive by the end of the week. Some women notice light pink or brown spotting for 1–2 days. Early symptoms may include breast tenderness, mild cramping, bloating and fatigue — feeling completely normal is also fine.

What parents need to know

You may have just seen a faint second line, or you're counting the days until your period is due and watching for every twinge. Week 4 is a quiet, invisible sort of week — a great deal is happening, and almost none of it shows.

What's happening at week 4?

Week 4 is a landmark moment: the blastocyst burrows into the lining of your uterus in a process called implantation. This is when your body officially becomes pregnant. As the embryo embeds itself, it starts releasing human chorionic gonadotropin (hCG) — the hormone that tells your ovaries to stop releasing eggs and triggers the production of oestrogen and progesterone to maintain the pregnancy.

Implantation explained

Implantation usually happens between days 6 and 10 after fertilisation — so typically around days 20–24 of a 28-day cycle. The blastocyst hatches from its outer shell and attaches to the endometrium (the uterine lining), then gradually burrows deeper. Blood vessels in the uterine wall begin connecting to the embryo, forming the early foundations of the placenta.

Implantation bleeding: what to expect

Some women notice light spotting around the time of implantation, often mistaken for a light period. It's usually pink or brown rather than red, lasts only 1–2 days, and is much lighter than a normal period. Not everyone experiences this — if you don't, that's perfectly normal too. If bleeding is heavy or accompanied by pain, speak to your GP.

Your pregnancy test turns positive

By the end of week 4, hCG levels are usually high enough to be detected by a home pregnancy test. The NHS recommends testing on the first day of your missed period for the most reliable result. Some sensitive early tests can detect pregnancy a few days before this. A faint line still counts as a positive — even a small amount of hCG confirms pregnancy.

Early pregnancy symptoms

You may start to notice some early signs of pregnancy this week, though not everyone does:

• Breast tenderness — your breasts may feel heavier, sore or tingly
• Mild cramping — similar to period cramps as the uterus adjusts
• Bloating — rising progesterone can cause digestive sluggishness
• Fatigue — your body is already working hard, even if you can't see it yet
• Heightened sense of smell — an early and often surprising symptom
• Frequent urination — increased blood flow to the kidneys kicks in early

Many women feel completely normal at week 4 — the absence of symptoms doesn't mean anything is wrong.

What's your baby doing?

At week 4, the embryo is about the size of a poppy seed — roughly 0.1–0.2mm. It's made up of three layers of cells that will become different body systems: the ectoderm (skin, brain, nervous system), mesoderm (muscles, bones, heart), and endoderm (digestive system, lungs). The amniotic sac and yolk sac are also beginning to form.

What should you do now?

If your test is positive, here's what to do next according to NHS guidance:

• Contact your GP or midwifery service to register your pregnancy and book your first antenatal appointment (booking appointment), usually around 8–10 weeks
• Continue taking 400mcg of folic acid daily until at least week 12
• Avoid alcohol, smoking and recreational drugs
• Check any medications you take with your GP — some are not safe in pregnancy
• Eat a healthy, balanced diet and stay hydrated

What's next?

Week 5 brings the first stirrings of the neural tube — the structure that becomes your baby's brain and spinal cord. This is why folic acid is so important in these early weeks. Our Week 5 guide will walk you through what to expect as your pregnancy progresses.