Pregnancy lasts approximately 40 weeks and is calculated from the first day of your last period, although conception will probably not have taken place until around two weeks afther this. Pregnancy is divided into three parts known as trimesters. The first covers weeks 1 - 13, the second trimester weeks 14 - 26 and the third trimester week 27 until birth.
During the first trimest, your baby will evolve from a cluster of cells into a recognizable fetus measuring about 3 inch (80 mm). All the major organs, muscles and bones will be formed. Until the placenta becomes mature enough to take over, your pregnancy is supported by maternal hormones, which also contribute to early symptoms such as nausea and tiredness. Although you may not look pregnant in the first trimester, you will almost certainly feel pregnant.