Early Development in Pregnancy

From the mother, the egg carries 23 chromosomes, the tiny structures carrying genes that guide development. The sperm contributes 23 chromosomes from the father. When these chromosomes unite, the new cell contains 46 chromosomes which hold the information that makes the baby a unique individual. Physical features, such as eye or hair color, are encoded within the chromosomes. The gender of the baby is determined by two specific chromosomes, referred to as the “sex chromosomes.” Two “X” chromosomes make a girl, an “X” and a “Y” make a boy. Because a woman can only give an “X” chromosome, it is the sperm that determines the sex of the baby.

Once fertilized, the egg begins to multiply in size. It implants in the endometrium and develops into an embryo. Cells from the embryo develop into the placenta and two sacs, one inside of the other, called the chorion and amnion. Cells from these structures can be obtained to learn if there are any chromosomal abnormalities in the developing fetus, which will be discussed in the Prenatal Testing Section.

The placenta is connected to the fetus by the umbilical cord.

The blood of the fetus does not mix directly with the mother’s blood. Rather, oxygen and nutrients cross from the mother’s circulation through a thin placental barrier to the baby’s blood. Likewise, carbon dioxide and waste products from the baby pass back to the mother.

It is important to remember that harmful substances such as alcohol, nicotine and certain drugs pass through the placenta to the baby. The fetus can be harmed by drugs, alcohol and other substances at any point during pregnancy, but it is especially vulnerable during the first three months of pregnancy when the organs are developing and forming. By the end of the third month all of the baby’s organs have been formed. Harmful habits and lifestyles can take a toll on the developing baby. If an expectant mother has any questions about changing her habits or taking any medication, she should discuss this with her provider early in the pregnancy or, preferably, before she gets pregnant.

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