A typical pregnancy spans 40 weeks, divided into three trimesters. The first trimester begins at conception. The Mayo Clinic categorizes fetal development in the first trimester over a period of 12 weeks, while other medical sources list the end of the first trimester at 13 weeks or even 14 weeks.
Using the Mayo Clinic’s timeline of fetal development, a woman actually is not pregnant during the first two weeks designated as the start of her pregnancy. When your due date is calculated, it begins at the first day of your last menstrual period, when you were not pregnant yet. Usually, conception occurs about two weeks after your last period began.
Weeks 1 and 2. These are the weeks after your last menstrual period before conception.
Weeks 3 and 4. In week three, the egg is fertilized, becoming a zygote with 23 chromosomes from each of the mother and father. Chromosomes determine a person’s sex, physical traits and even some intelligence and personality traits. Week four sees the zygote developed into a blastocyst and attaching to uterine wall. This attachment is called implantation. The placenta begins to form at this time as well.
Weeks 5 and 6. During the embryonic period of week five, the baby begins brain, heart and spinal cord development. Other organs will begin to develop as well. Week six is a busy time, as the baby forms basic facial features and tiny buds, the beginnings of arms and legs. The body now is curved into a C-shape as well. The neural tube in the back closes, and the heart pumps blood.
Weeks 7 and 8. At week seven, the baby’s head continues development, with brain growth and some facial features filling in. The arm buds continue growth as well. In the eighth week, your baby has eyes, a nose and an upper lip. Ears are forming and fingers are emerging as limbs grow. The baby is still tiny, about one-half inch long at the end of eight weeks.
Weeks 9 and 10. Weeks nine and 10 are busy. Facial features continue to develop, including eyelids. Bones form in the arms, which now even bend at the elbows. Details continue to fill in with your rapidly developing baby. There are now toes, a neck and a rounded head.
Weeks 11 and 12. At week 11, the baby may be about 2 inches long and weigh as much as 8 grams, states the Mayo Clinic. The baby is now technically termed a fetus. Genitals have formed, and the arms and legs are longer and thinner. Red blood cells begin to form in the liver as well. Week 12 marks the end of the first trimester. Your baby now has fingernails and an identifiable human profile. He can make a fist, and the beginnings of teeth appear.