Pregnancy is divided into three trimesters. The first trimester covers the time from conception to week 12 and is a vital stage in your baby’s development when all of the major organs begin to form. The first trimester is also a period when your baby is most vulnerable to harm, so it is important to look after your health, eat well and avoid alcohol and drugs.
Once the egg has been fertilised by the sperm, it travels along the fallopian tube towards the uterus where it implants itself. This process triggers the release of hormones, which sparks changes in your body to prepare for pregnancy. By week four of the first trimester, the fertilised egg, which is now a cluster of living cells, will separate into two. One part will form the embryo and amniotic sac and the other part will form the placenta, which will supply your growing baby with nutrients and oxygen via the umbilical cord. The development of blood and blood vessels, bones, muscles and major internal organs has also begun. Some blood vessels will form the umbilical cord and connect to the placenta, providing a vital link between you and your baby.
Around the size of a coffee bean, your baby is recognisable as a human being even at this early stage of the first trimester. The cluster of living cells has now developed into an embryo. Your baby has a neck and head, a brain and a heart that is already beating, although it has only two chambers rather than four. An ultrasound scan may even be able to pick up a heartbeat at this early stage. Tiny buds that will become arms and legs are already in place. The beginnings of a digestive system, kidneys, and a liver have all started by this stage of the first trimester.
Your baby is now the size of a grape. His/her features, especially the eyes and mouth, are becoming more distinct. The eyes are covered by an intact skin, which will eventually split to become eyelids. The limbs elongate and elbows and knees begin to appear. The internal organs are continuing their development – the heart has started its vigorous pumping. Your baby will now be about 2.5cm long at this stage of the first trimester.
Your baby is now about the size of an apricot. Development is well underway with fingers and toes becoming more distinct and the facial features developing more. Your baby will be moving around and is about 4.5cm long and weighs about 5g.
Your baby’s head is now fully formed. Eyes and eyelids are complete. Fingers and toes are fully formed; arms have grown and nails have started to grow. Your child’s ovaries or testicles are distinguishable but not fully developed so a scan at this stage cannot determine the sex of the baby. The ribs and spine are beginning to harden into bone by the end of the first trimester. Your baby’s length is now about 6.5cm, but weighs only 18g.