Trimesters - First, Second and Third
The gestational period of a pregnancy is divided into sections of time called trimesters. Officially the trimester definition is "a period or term of three months." A full-term pregnancy lasts nine months. When divided into sections of three, the gestation period is three trimesters.
For fetus developmental purposes these sections of time in the gestational period are further divided into trimester weeks. Your unborn baby goes through drastic changes weekly and as you progress in your pregnancy the viability of your baby increases substantially with each passing week if your baby were to be born premature.
The First Trimester
The first trimester officially begins well before you know you're pregnant. During the trimester weeks one and two of your first trimester you're actually not pregnant. But they're counted as part of trimester one because your doctor or midwife will use the date of your last period to estimate your due date.
Conception occurs in week three. This happens when an egg is released into one of your fallopian tubes and a sperm fertilizes that egg. The fertilized egg is called a zygote and will have 23 chromosomes from you and 23 chromosomes from your partner. These determine eye and hair color, your baby's sex and other traits.
The zygote travels to the uterus where implantation occurs. It's constantly changing and developing and the inner cells change to an embryo while the outer cells become a special membrane that protects and nourishes the embryo. By the end of week four and after implantation, a positive pregnancy test is possible.
The embryonic period starts the fifth week and your baby's organs like heart, spinal cord and brain begin to form. In the sixth week your baby's heart is pumping blood. Around eight weeks your baby will start to move, but she'll be too tiny for you to feel those movements. Week eleven marks your baby's transition from embryo to fetus and by week 12, the end of the first trimester, your baby's face will look human.