Your pregnancy and baby guide
You and your baby at 4 weeks pregnant
Open all pages about Your pregnancy and baby guide
- Secrets to success
- Am I pregnant?
- Early days
- Week by week
- Preparing for the birth
- Work out your due date
- Tests scans and checks
- Your pregnancy (antenatal) care
- Your health and wellbeing
- Existing health problems
- Common pregnancy ailments
- Pregnancy-induced conditions
Labour and birth
- The start of labour
- The birth
- Emotions and worries
- Premature babies
- How to breastfeed
- Breastfeeding problems
- Lifestyle and breastfeeding
- Bottle feeding
- Newborn screening tests
- Newborn essentials
- New parents
- New mums
- Twins and multiples
Babies and toddlers
- Weaning and solid foods
- Baby health and care
- Spotting signs of serious illness
- Reflux in babies
- How to take a baby's temperature
- Reducing the risk of SIDS
- Treating a high temperature
- Sleep problems in children
- Coughs, colds and ear infections
- Diarrhoea and vomiting
- Infectious illnesses
- Children's medicines
- Looking after a sick child
- Serious conditions and special needs
- Constipation in young children
- Your baby's height and weight
- Baby health and development reviews
- Leg and foot problems in children
- Learning, play and behaviour
- Safety and accidents
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 – find out about units of alcohol and tips on avoiding alcohol in pregnancy
- 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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Page last reviewed: 04/10/2019
Next review due: 04/10/2022