Pregnancy is measured in trimesters from the first day of your
last menstrual period, totaling 40 weeks. The first trimester of
pregnancy is week 1 through week 12, or about 3 months. The
second trimester is week 13 to week 27. And the third trimester
of pregnancy spans from week 28 to the birth. Your baby will
change from week to week. For more information about how your
baby is changing each month and about what tests you might think
about having, see the Reference Interactive Tool: From Embryo to
Baby in 9 Months Reference . During the week after
fertilization, the fertilized egg grows into a microscopic ball
of cells (blastocyst), which implants on the wall of your
uterus. This implantation triggers a series of hormonal and
physical changes in your body. The third through eighth weeks of
growth are called the embryonic stage, during which the
Reference embryo Opens New Window develops most major body
organs. During this process, the embryo is especially vulnerable
to damaging substances, such as alcohol,