4 Weeks Pregnant: Baby Development

At 4 weeks pregnant you may have only just taken a positive pregnancy test — and yet an enormous amount is already happening inside you. Your embryo is tiny, but the biological processes under way at this stage lay the entire foundation for your baby's development over the coming months. Here's what is actually going on at week four.

Baby Size at 4 Weeks
What Is Happening Inside
Implantation
The Neural Tube
The Placenta and Yolk Sac
What You Might Be Feeling
When to Contact Your GP
Frequently Asked Questions

Baby Size at 4 Weeks

Your embryo is approximately the size of a poppy seed — roughly 0.1 to 0.2 mm. That is almost unimaginably small. Despite this, a great deal of complex cellular organisation is already underway. By the end of this week your embryo will consist of three distinct layers of cells that will eventually become every organ and tissue in your baby's body.

What Is Happening Inside

At four weeks, the embryo has three primary layers called the ectoderm, mesoderm, and endoderm. Each one has a specific developmental role:

Ectoderm — this outer layer will go on to form the nervous system, skin, hair, and sensory organs including eyes and ears.
Mesoderm — the middle layer will develop into the heart, blood vessels, muscles, bones, and kidneys.
Endoderm — the inner layer will form the digestive system, lungs, liver, and pancreas.

The amniotic sac, which will cushion your baby throughout pregnancy, is also forming at this stage. The yolk sac — visible on very early scans — is providing nutrients to the embryo before the placenta is fully established.

Implantation

By around four weeks, the fertilised egg (now a blastocyst) has travelled down the fallopian tube and embedded itself into the lining of your uterus. This process is called implantation and it is what triggers your body to begin producing human chorionic gonadotrophin (hCG) — the hormone that pregnancy tests detect. Some people notice very light spotting or mild cramping around the time of implantation, though many notice nothing at all. Both are completely normal.

Implantation bleeding, when it occurs, is typically very light — often just a few spots of pink or brown discharge — and lasts only a day or two. It is not the same as a period. If you experience heavier bleeding, more prolonged spotting, or significant pain, contact your GP or Early Pregnancy Unit (EPU) for advice.

The Neural Tube

One of the most important developments happening right now is the beginning of neural tube formation. The neural tube will eventually become your baby's brain and spinal cord. It starts forming from around day 22 to day 28 of pregnancy — which corresponds to around weeks four and five.

This is why folic acid supplementation matters so much in early pregnancy, ideally started before conception and continued through at least the first 12 weeks. Adequate folate supports healthy neural tube closure. The NHS recommends 400 micrograms of folic acid daily for most women; your GP or midwife may recommend a higher dose if you have certain health conditions or a family history of neural tube differences.

The Placenta and Yolk Sac

Two temporary but essential structures are developing alongside your embryo at four weeks:

The yolk sac is visible on early ultrasound scans and acts as the embryo's first source of nourishment. It produces nutrients and red blood cells until the placenta is ready to take over. Seeing a healthy yolk sac on a very early scan is a positive sign that the pregnancy is developing in the right place and getting what it needs.

The placenta is in its earliest stages of formation at four weeks. It develops from some of the same cells as the embryo, growing rapidly into the uterine wall. By around ten weeks it will have taken over from the yolk sac as the primary source of oxygen and nutrition for your baby, a role it will maintain until birth. The placenta also produces hormones — including progesterone — that support the pregnancy and are responsible for many of the symptoms you may be experiencing.

What You Might Be Feeling

Physically, week four is often when the first pregnancy symptoms begin — though not everyone feels them straight away. You might notice:

Breast tenderness or sensitivity
Mild tiredness or fatigue
Bloating or a feeling of fullness
Light spotting (implantation bleeding)
A heightened sense of smell
Mild nausea, though full morning sickness usually kicks in from around six weeks
Frequent urination
Emotional sensitivity or mood changes

Many people feel no symptoms at all at four weeks and still have perfectly healthy pregnancies. The absence of symptoms at this stage does not indicate a problem. Equally, feeling very symptomatic is not a guarantee that everything is progressing well — symptoms are caused by hormones, and hormone levels vary naturally between individuals.

When to Contact Your GP

If you haven't already, contact your GP as soon as you have a positive pregnancy test so they can refer you for your booking appointment. This appointment, with a midwife, usually happens between 8 and 12 weeks and is when your full antenatal care plan is established.

Contact your GP, midwife, or Early Pregnancy Unit sooner if you experience:

Heavy bleeding (soaking a pad) at any point
Severe or one-sided abdominal pain
Shoulder tip pain alongside bleeding or cramping, which can be a sign of ectopic pregnancy
Dizziness, faintness, or feeling very unwell alongside any of the above

If you are ever unsure whether your symptoms need assessment, you can call NHS 111 for advice at any time.

Frequently Asked Questions

### How big is my baby at 4 weeks?

Your embryo is about the size of a poppy seed — approximately 0.1 to 0.2 mm. At this stage it consists of three layers of cells that will develop into every organ and structure in your baby's body over the coming weeks and months.

Can I see my baby on a scan at 4 weeks?

At four weeks it is almost certainly too early for a scan to show anything meaningful. Even with a transvaginal ultrasound, the embryo is far too small to be seen clearly. Most early scans in the UK are performed from around six weeks, when a gestational sac and yolk sac become visible. Heartbeat detection typically happens from around six to seven weeks. If you have concerns about early pregnancy, contact your GP or Early Pregnancy Unit (EPU) rather than arranging a scan at this stage, as results may be inconclusive and could cause unnecessary worry.

Should I be taking any supplements at 4 weeks?

Yes. Folic acid (400 micrograms per day) is the most important supplement at this stage, and ideally you will already be taking it. Vitamin D (10 micrograms per day) is also recommended throughout pregnancy by the NHS. Some women take a combined pregnancy multivitamin rather than individual supplements — this is fine as long as it contains the recommended amounts and does not include vitamin A (retinol), which should be avoided in pregnancy. Speak to your midwife or GP if you are unsure which supplements are right for you.

Always speak to your midwife or GP for personalised advice about your pregnancy.