Your baby at 4 weeks
In weeks 4 to 5 of early pregnancy, the embryo grows and develops within the lining of the womb.
The outer cells reach out to form links with the mother's blood supply. The inner cells form into 2, and then later into 3, layers.
Each of these layers will grow to be different parts of the baby's body:
- the inner layer becomes the breathing and digestive systems, including the lungs, stomach, gut and bladder
- the middle layer becomes the heart, blood vessels, muscles and bones
- the outer layer becomes the brain and nervous system, the eye lenses, tooth enamel, skin and nails
In these early weeks of pregnancy, the embryo is attached to a tiny yolk sac that provides nourishment.
A few weeks later, the placenta will be fully formed and take over the transfer of nutrients to the embryo.
The embryo is surrounded by fluid inside the amniotic sac. It's the outer layer of this sac that develops into the placenta.
Cells from the placenta grow deep into the wall of the womb, establishing a rich blood supply. This ensures the baby receives all the oxygen and nutrients it needs.
You at 4 weeks
Conception usually takes place about 2 weeks after your last period, around the time you release an egg (ovulate).
In the first 4 weeks of pregnancy, you probably will not notice any symptoms.
The first thing most women notice is that their period does not arrive, or they may have other signs and symptoms of pregnancy, such as breast tenderness.
Most women confirm the pregnancy with a pregnancy test.
You can work out the date when your baby is due. This date may be changed when you have an ultrasound scan.
Things to think about
- what to expect on your NHS pregnancy journey
- there's help and support if you're a teenager
- avoid drinking alcohol when you're pregnant or trying to get pregnant
- how physical and emotional changes in pregnancy can affect your relationships
The Start4Life site has more about you and your baby at 4 weeks of pregnancy.
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Media review due: 5 February 2023