Early Stages of Pregnancy

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Pregnancy is traditionally divided into three trimesters. Each week of pregnancy involves different developmental changes to the developing fetus as well as changes in the mother. After conception and during the early stages of pregnancy, vital hormonal changes such as the following are taking place:
- Extreme, unexplained fatigue is one of the first signs of pregnancy
- Food aversions
- A sensitivity to smells
- Nausea and vomiting, sometimes called morning sickness, although it may occur at any time of the day.
Even with these signs, some women do not realize at first that they are pregnant.
Early Stages of Pregnancy and Conception
In the first week following conception, the fertilized egg implants itself in the uterine wall, which triggers the hormonal and physical changes that women experience. The amniotic sac develops during the first week, cushioning the embryo. The placenta also develops during this first week.
During the first month, the developing embryo goes from being a two-cell organism to something the size of a kidney bean. All of the major organs develop. From weeks three through eight, the developing fetus is particularly vulnerable to the effects of toxins such as alcohol.
Early Stages of Pregnancy and Gestation
It is during the ninth week of gestation that the embryo is called a fetus, having reached approximately an inch in size. The woman's uterus has expanded during this time, reaching the size of an average grapefruit.