You at 4 weeks pregnant
The fertilised egg moves down your fallopian tube to the uterus, where it implants itself in the endometrium. This can take 3-10 days. Some women have a small amount of bleeding - or 'spotting' - around the time of implantation.
You might not notice any changes in your body such as sore breasts and tiredness, but by the end of the week you'll probably have missed your period and could be wondering if you're pregnant.
Your baby when you're 4 weeks pregnant
From the moment of conception, the fertilised egg - called a blastocyst - starts dividing itself. By the end of this week, it's a ball of about 200 cells and is about 4-5 mm across. Inside the ball, three layers are forming:
- The ectoderm - this becomes your baby's nervous system, brain, hair and skin.
- The endoderm - this becomes the gut and other internal organs.
- The mesoderm - this develops into the skeleton, blood systems and muscles.
The outside of the blastocyst divides into two parts:
- The outside part sends out little tentacles - called chorionic villi - which bury into your endometrium. The chorionic villi develop into the placenta.
- The inside part becomes the amniotic sac - the protective bubble filled with fluid where the embryo develops.
Identical, or monozygotic, twins are conceived when a single sperm fertilises an egg. At a very early stage in the cell division process, the fertilised egg divides into two and starts forming two babies. Identical twins have the same genes, so they're the same gender.