Your 9 months Pregnancy 4 weeks pregnant in pictures You are now in your 4th week of pregnancy! 1 of Ad break How big is your babyAt this early stage of life, your baby-to-be is called an embryo, and is around the size of the head of a pin. Now find out how your baby is growing… How is your baby growing? The cells have been rapidly dividing and there are now three layers: The outer layer - which will develop into your baby's brain, skin, nervous system, hair, nails and teeth. The middle layer - which will develop into your baby's heart and blood vessels, bones, muscles and reproductive organs. The inner layer - which will develop into your baby's digestive system, liver, lungs and bladder. The shape of your baby-to-be is changing, from a ball of cells to a leaner, flatter shape. Now find out how your body is changing… What’s happening to your body? Some mums-to-be have a strong feeling that they're pregnant before they do a test. You may find your breasts feel slightly enlarged or tender and that your nipples are particularly sensitive. Other mums seem to have a sixth sense, saying that they just 'felt different'. Learn more about what symptoms to expect this week and check out our week by week pregnancy guide... Find out about when you're 3 weeks pregnant or move on to 5 weeks pregnant. By Sara Mackenzie Comments Latest on MadeForMums Princess Marie-Chantal’s touching support for Kate Middleton (Exclusive) Danielle Lloyd's luxury pram pick for baby no 4 Rochelle Humes' divisive method for clearing her baby's bunged up nose Viral wedding invitation tells guests: "Breastfeed in the toilets!"