Your 9 months Pregnancy 3 weeks pregnant in pictures You’re into the 3rd week of your pregnancy, here’s what’s happening 1 of Ad break How big is your baby?This week the newly united sperm and egg make a home for themselves in your womb. Size wise we're still talking almost microscopic size sperm and egg. It will still be a few weeks before your the cells grow to a visible size.Now find out what your baby is doing… How is your baby growing?The mass of cells, which has been steadily dividing since fertilisation, is called the ‘blastocyst’. A very scientific name for something that you will soon be calling ‘baby’. About a week after fertilisation – called week 3 of your pregnancy because it is about 3 weeks from your last period – the blastocyst is ready to burrow its way into your womb lining (endometrium). This is called ‘implantation’ and can be one of the most tricky points in early pregnancy. Now find out what’s happening in your body… What’s happening to your body?In order to make the lining of the womb as healthy as possible, the blastocyst now starts to produce a hormone referred to as hCG (human chorionic gonadotrophin), which triggers other hormonal activity. It’s this hCG that can be detected in urine samples when you’re doing a pregnancy test. Once implantation has occurred, the placenta will start to form. The placenta not only creates the pregnancy hormones needed for you to physically change during pregnancy, but also acts as a feeding mechanism and protects the foetus from your immune system. Just as blood delivers oxygen and other vital nourishment to parts of your body, so blood will do the same job for your baby, via a network of vessels in the placenta. Already, you might notice some early signs of pregnancy, such as feeling extremely tired, even if you don’t know yet that you are expecting a baby. Learn more about what symptoms to expect this week and check out our week by week pregnancy guide... Find out what went on week 2 of your pregnancy, or what to expect when you're 4 weeks pregnant. By Laura Lee Davies Comments Latest on MadeForMums Harry Kane, Kate Goodland and child - family facts Katie Piper opens up about her post-baby body - 6 months after having Penelope Cough syrup recalled over warning that one of its ingredients goes mouldy Katie Piper combats mum guilt: 'I'm certainly not gonna torture myself over things'