The weeks of pregnancy are counted from the first day of your last period. Normally, your baby is born at 40 weeks. You aren't even pregnant during week one, because the 40 weeks is based on when you had your last period, not when a sperm joined or "fertilized" the egg. Usually the pregnancy starts at the end of week two or the beginning of week three, depending on when your body ovulates.
How does a pregnancy start?
Fertilization is the beginning of pregnancy. About 14 days after the first day of your last period, your ovaries release an egg. If you have sex a few days before or after your egg is released, a sperm can fertilize the egg. Fertilization happens in the tubes that connect your ovaries with your uterus. These are the fallopian tubes. The fertilized egg is called a zygote.
From Fertilization to Implantation
Once a month, an egg is released from your ovary into your fallopian tube. After sex, sperm move from the vagina to the fallopian tubes, where a sperm may fertilize the egg. The cells of the fertilized egg keep dividing as the egg moves to the uterus, where it implants in the wall.
What happens after fertilization?
The fertilized egg attaches to the lining of your uterus and begins to grow into 2 separate things:
The embryo is the part of the fertilized egg that eventually becomes a baby. It is considered a fetus starting at about 10 weeks.
The placenta develops from the fertilized egg but does not become part of the baby. It is an organ that provides nourishment to the growing embryo. One side of the placenta is attached to the inside of your uterus.
After a few weeks, the umbilical cord grows out of the other side of the placenta. The cord connects the embryo to the placenta. Blood from the embryo moves through the cord into the placenta. In the placenta, the embryo's blood picks up oxygen and nourishment from your blood. Then the oxygen and nutrient rich blood moves back through the cord into your baby. Right after your baby is delivered, the placenta comes off your uterus and is delivered. The placenta then is called the afterbirth.
The amniotic sac develops and surrounds the embryo. It fills with fluid for the embryo to float and grow in. This fluid helps protect the embryo from injury.
Milestones in the development of your fetus:
5 weeks: the heart starts to beat and most other organs begin to develop, followed by the brain and spinal cord
10 weeks: the embryo is considered a fetus
12 weeks: most organs are formed
14 weeks: doctors can tell the sex of the fetus
16 to 20 weeks: you may be able to feel movement
24 weeks: the fetus has a chance of survival outside the uterus
The fetus’s lungs continue to develop until near the time of delivery. The brain continues to develop throughout pregnancy and the first year of life after birth.
There are two types of twin pregnancies.
Fraternal twins happen when 2 eggs are released, and both eggs get fertilized by 2 different sperm. Each becomes an embryo with its own unique set of genes.
Identical twins happen when a single fertilized egg splits into 2 separate zygotes. Because the one egg was fertilized by one sperm, the 2 embryos share the same set of genes.
Triplets and other multiple pregnancies also happen the same ways.