Mother and Baby

Pregnancy symptoms week 3: expert advice and what to expect

Section: Week by Week
3 weeks pregnant

At three weeks pregnant, it is time for the sperm to meet the egg. As we mentioned in week two, your partner’s sperm goes on a very difficult journey to fertilise your egg, but if it’s made it and you’ve conceived, this is the week when your baby will first start to form.

At week three, you're in the first month of your pregnancy and you still have eight months to go until you can meet your little baby! 

Week three pregnancy

In this article:

How big is my baby at three weeks pregnant?

Right now, your baby has transformed from a fertilised egg (a single cell called a zygote) to a ball of around 100 cells about the size of a pinhead. This ball of cells is officially referred to as a blastocyst.

Baby the size of a pin head at three weeks

Common symptoms of pregnancy to look out for:

What symptoms and signs of pregnancy do you need to look out for in these early days and weeks?

  1. A heightened sense of smell
  2. Abdominal pressure (like period cramps)
  3. Metallic taste in mouth
  4. Nausea/morning sickness
  5. Implantation bleeding
  6. Missed period
  7. Positive home prengancy test

More early pregnancy symptoms and expert advice from the obvious to the more subtle changes. 

What is my baby doing?

Within hours of being fertilised, your tiny egg will start to travel from your fallopian tube to your uterus, and the fertilised egg (now medically referred to as the zygote) will divide into 16 identical cells.

This journey can take up to six days, so implantation probably won’t happen until week four.

The inside of the cell mass will become the embryo itself and the amniotic sac. The outer cell mass will become the placenta.

What's happening in my body?

Body at three weeks pregnant From the outside, it probably won’t feel like much is happening right now, but if you’ve timed things right, that precious blastocyst is starting out on its journey and heading for your uterus.

Just after the egg is released from the follicle it came from, it will be replaced by a yellow group of cells called the corpus luteum.

This, in turn, produces enough of the pregnancy hormones progesterone and oestrogen, to support your future baby for the next ten weeks, until the placenta is ready to take over.

At this point, you still probably won’t get a positive pregnancy test, but in the next few weeks, once the blastocyst has made its way into the uterus, the cells of the developing placenta will make human chorionic gonadotropin (hCG). This surges in your first trimester and tells your ovaries to stop producing eggs and start making more progesterone and oestrogen.

Once hCG is in your system, home pregnancy tests will pick it up - some home tests these days are sensitive enough to be able to give a positive result after as little as 3 or 4 weeks of pregnancy. 

The amount of pregnancy hormone hCG in your body may not yet be high enough for a pregnancy test to detect it.

However, the hormone doubles every 48 hours and if you get a negative result you can always take a second a few days later.

What should I be doing at this stage?

Healthy balanced diet at three weeks pregnantYou need to make sure you are eating healthily and getting the right nutrients and vitamins when trying to conceive or during the early stages of pregnancy.

  • Up the protein: Your baby will need extra protein to help it grow, so try and make sure you get three servings of protein a day in these first few weeks.
  • Make sure you’re getting enough calcium: It might seem obvious, but if you’re not getting enough calcium in your diet when you’re pregnant, your baby will start taking it from your bones! Now’s the time to up the intake – foods like Greek or frozen yoghurt will help. If you’re still struggling, it might be worth looking for a calcium supplement

Read more: Your go-to guide to taking supplements while pregnant

Things to think about this week...

Happy mum at three weeks pregnant

Work out your due date

If you're having trouble working it out on your fingers, try out our due date calculator. All you need to know is the first day of your last period and how many days your menstrual cycle is!

Educate yourself! 

It's time to start reading all about what to expect in your first trimester. You'll find out what your body and your baby will be doing in the next 12 weeks!

Keep taking your vitamins 

Taking prenatal vitamins will benefit your baby's development and nourish your pregnancy body and health. Adding a supplement of 400 micrograms of folic acid to your diet will help the foetus to develop healthily. 


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  • Author: Lorna White Lorna White
  • Job Title: Digital Writer

Lorna is the digital executive and regular contributor for Mother&Baby. After running the Yours magazine website which specialises in content about caring for kids and grandchildren, she has now brought her expertise to the UK's #1 leading pregnancy and parenting magazine. Lorna specialises on a range of topics from potty training and nutrition, to everything and anything that will keep your tot occupied!