Development

• Once the egg is fertilized, the cells begin to divide. As the cells, called the morula, divide, they move towards the uterus. A cluster of these cells becomes the embryo while the others form the placenta and the membranes that surround the growing embryo and later, the fetus. The morula becomes a fluid-filled sphere called a blastocyst. The cells in the center of the blastocyst become the embryo and, nine days after egg fertilization, the blastocyst attaches itself to the uterine wall. Once this implantation occurs, the placenta forms to provide nourishment to the growing embryo. The placenta produces the hormone HCG, which can be detected in pregnancy tests.
  
  

Twins and Multiples

• In the first 20 hours of fertilization, the bundle of cells that develop, called a zygote, may split. If this happens, identical twins form. The latest date an embryo can split to become twins is 13 days after conception. The embryo is typically implanted into the lining of the uterus by day thirteen. If the embryo divides after the thirteenth date, Siamese, or conjoined twins, will form.
  
  

Early Pregnancy Symptoms

• The physical symptoms of pregnancy can occur as early as the first two weeks. Symptoms can include dizziness, nausea, frequent urination, fatigue, breast tenderness and mild abdominal cramps. Mood swings and weepiness are also a symptom of pregnancy. Many women do not have symptoms within the first two weeks of conception; often, a missed menstrual cycle is the first real symptom of pregnancy in the early weeks of the first trimester.