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!
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.
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?
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.
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.
Avoid getting your hair dyed: It's a good idea when you're pregnant to avoid having any sort of chemicals touch your scalp. Talk to your hairdresser about chemical-free alternatives they can recommend.
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!
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.
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!