4 weeks pregnant

You and your baby's development

What's happening in week 4 of your pregnancy

It's still early days, many still won't know they are pregnant.

At a glance

  • Your growing baby is not even 3mm long yet
  • Their brain and central nervous system are beginning to take shape
  • You might feel some pregnancy symptoms at 4 weeks
4 Weeks Pregnant

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How big is my baby at 4 weeks pregnant?

Although a fertilised egg may have implanted in your womb just two weeks ago, if the first day of your last period was four weeks ago, you are four weeks pregnant.

At this stage of pregnancy your baby is known as an embryo. They get their energy and nourishment from a yolk sac, until the placenta takes over in a few weeks. Your growing embryo is not even 3mm long yet and still not much bigger than an apple seed. Despite the tiny size there’s plenty going on as the embryo splits into three different sections!

In one section the brain and central nervous system are already beginning to take shape as their neural tubes develop. In the other two sections, the heart and circulatory system are already beginning to form and the lungs and intestines are in the very early stages of development.

How amazing is it that all this is going on totally undetectable to the outside world? (and maybe even you at this point). 

Did you know:

  • Babies start to sleep from 4 weeks after conception 

  • Your fertilised egg is about the size of an apple seed 

  • An embryo's heart begins to beat just 3 weeks after conception 

  • Before the first month of the first trimester, baby's hearing is forming 

You at 4 weeks pregnant

You might feel some pregnancy symptoms at four weeks, but there is every chance you will put them down to your period being about to arrive.

Your boobs might have that 'time of the month' feeling, and you might even have some spotting or light bleeding.

As soon as you know you are pregnant, please get in touch with your local maternity unit to book your first appointment. This is called your ‘booking appointment’, and you will meet a midwife for the first time.

Ideally you should be seen by a midwife by the time you are 10 weeks pregnant, or as early into your pregnancy as possible.

You can make your first appointment quickly and easily by self-referring directly to your local maternity unit or visiting your GP.

Check out our newly pregnant page to find out what to expect during your first brush with maternity services.

You can also work out an approximate due date using our pregnancy due date calculator. 

Did you know:

  • Due date is calculated by counting from the first day of your last period
  • As soon as you're are pregnant, your blood flow around your body increases
  • Just days after conception, your breasts are already preparing for milk production

What to think about at 4 weeks pregnant

When should I take a pregnancy test?

Pregnancy tests confirm your pregnancy by detecting the hormone hCG (human chorionic gonadotrophin) in your urine. This hormone will be present in your body around six days after fertilisation, and it takes about two weeks from conception for hCG to reach a level that will be picked up by a pregnancy test. 

This usually occurs around the time your period is due, so this is an ideal time to pee on that stick! It’s important to remember that pregnancy testing at home may not always be 100% accurate though especially if you don’t follow the test instructions and test at the right time. If you do get a negative result, it’s sometimes worth doing another test a few days or week after your missed period to see if it comes back positive as the hCG levels increase.

As soon as you've taken a positive pregnancy test, you need to visit your GP so you can get signed up with your local maternity services team.

Looking after your teeth is really important in pregnancy too, as expectant mums can sometimes be more prone to problems like bleeding gums
  • a MATB1 certificate issued by your midwife or GP
  • a valid prescription maternity exemption certificate (MatEx)

Even at this very early stage of pregnancy, there are some important things to be aware of. 

Implantation bleeding is something that can occur in the very early days and weeks of pregnancy. 

Signs and symptoms at 4 weeks pregnant

Implantation bleeding

If around the 4 week mark of your pregnancy you notice a little bleeding, or a pink or brownish discharge, chances are it is implantation bleeding this occurs when a fertilised egg attaches itself to the lining of the uterus in order to begin growing. It’s thought about one third of women experience implantation bleeding. The difficulty is that it happens at a similar time in your cycle to menstruation, so it’s often confused with having an early period. This very early sign of pregnancy is not usually a reason to be concerned; it is perfectly normal and no risk to the developing baby. However, if you are also experiencing lower abdominal pain, nausea, dizziness or vomiting you should see your doctor.

Sore boobs

Having tender boobs early in your pregnancy is a common early symptom of pregnancy and happens as a result of a hormones surge to help support your growing baby. You’ll produce more oestrogen and progesterone – much like in the run up to your period. You may also notice early in your pregnancy blueish veins just below the skin's surface on your boobs. These are carrying nutrients and fluids from you to your baby. The best way to beat boob soreness during pregnancy is with a good, well-fitting bra.

Watch our video below on: Knowing you are pregnant

NHS Choices logo Video: Is the pregnancy test accurate?

At a glance

  • Your growing baby is not even 3mm long yet
  • Their brain and central nervous system are beginning to take shape
  • You might feel some pregnancy symptoms at 4 weeks

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