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

7 min read

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.

Baby's size	About a poppy seed, around 2mm long
Implantation timing	Days 6–10 after fertilisation
When to test	First day of your missed period
Folic acid	400mcg daily until at least week 12
Booking appointment	Usually around 8–10 weeks

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.

Cute baby engaged and curious while looking at a book indoors.

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. Spotting can have other causes too, so get urgent advice for any bleeding in pregnancy, even if it's light. Call 999 if the bleeding is heavy or comes with severe tummy pain, shoulder pain, or feeling sick, faint or dizzy. See When to get help below.

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, around 2mm long. 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, this is what to do next according to NHS guidance:

The Crib

What to do after a positive test

Your next steps at week 4, 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

Sources: NHS, Tommy's, NCT. General information, not medical advice.

When to get help

Some symptoms in pregnancy can be a sign of something serious and need checking quickly. This is who to contact, and how soon.

Call 999 if:

you're bleeding from your vagina and the bleeding is heavy (soaking a period pad soon after putting it on), or you also have severe pain in your tummy, pain in your shoulder, or feel sick, faint, dizzy or lose consciousness
you have a sharp, sudden and intense pain in your tummy together with feeling very dizzy or fainting, feeling sick or looking very pale, as this could mean an ectopic pregnancy has ruptured (you can also go straight to A&E)
you have severe difficulty breathing (you're gasping, choking or not able to get words out) or your chest feels tight or heavy
you have pain and swelling in one leg and you also feel short of breath or have chest pain. A blood clot in the leg can travel to your lungs, which can be life-threatening
you have possible signs of sepsis, such as breathing very fast, being confused or having slurred speech, blue, pale or blotchy skin, a rash that does not fade when you press it, or a very high or very low temperature. Being pregnant makes sepsis more likely
you do not feel you can keep yourself safe (a mental health emergency is as serious as a physical one)

Always call 999 if you think it's a life-threatening emergency, no matter what stage of pregnancy you are.

Contact your GP or early pregnancy unit right away if:

you have light bleeding or spotting, even if you have no pain or only mild tummy pain
you have vaginal discharge that's unusual for you, or fluid leaking from your vagina
you have tummy pain low down on one side, or pain in the tip of your shoulder (where your shoulder ends and your arm begins). Symptoms of an ectopic pregnancy usually start between 4 and 12 weeks, so get advice right away, even if you have not had a positive pregnancy test yet
you have stomach pain that is severe or does not go away after you've rested for 30 to 60 minutes, or stomach pain with regular cramping or tightenings, lower back pain, or pain when you pee
your breathlessness comes on suddenly, you're out of breath when you're lying down, or you're breathless and also have chest pain or feel dizzy
you have pain or swelling in one leg, usually in the calf or thigh. This could be a blood clot (deep vein thrombosis, or DVT), which is more likely when you're pregnant
you have a high temperature (38C or above), or you feel hotter than usual or shivery. A high temperature can be a sign of an infection, so it's important to get it checked

Also contact your GP or early pregnancy unit right away if you have tummy pain and you need to pee suddenly or more often than usual, or your pee looks cloudy, pink, red or brown. Pain when you pee, pain or pressure in your bottom when you poo, or diarrhoea can also be signs of an ectopic pregnancy, so get medical advice if you have any of these.

If you cannot reach them, or you're not sure what to do, call NHS 111.

Speak to your GP or midwife if:

you're worried about your mental health, or stress, anxiety or low mood are affecting your day-to-day life. If you need urgent help for your mental health, call 111 and select the mental health option, or ask for an urgent GP appointment

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.

Sources & further reading

General information, not individual medical advice. Speak to your GP, midwife or health visitor about your baby’s needs.