Pregnancy Terms
A Glossary for Moms-to-Be
Gestational Sac and Placenta
The gestational sac is like a sturdy, water-filled balloon. Two membranes, the outer one called the chorion and the inner one called the amnion, enclose the gestational sac. Inside the sac is amniotic fluid, and floating within that are the baby and the umbilical cord. The umbilical cord (coming out of what will later be the baby's belly button) connects the baby to the placenta, a thickened part of the outer membrane. The placenta, which has blood flow from both the mother and the baby, is where mother and baby "communicate." Even though the blood systems of mother and baby are kept completely separate – never mixing together – oxygen and nourishment transfer from the mother's bloodstream to the baby's in the placenta. The mother's blood enters from the outer surface of the placenta (which is attached to the mother's body), and the baby's comes from the umbilical cord (which is attached to the baby). Nutrients and oxygen transfer from the mother's blood to the baby's blood to meet the needs of the growing baby.
Gestational Age
This is the traditional method of measuring the length of a pregnancy from the first day of the last menstrual period (LMP). For most of human history, the LMP was the only identifiable marker for the start of pregnancy. Now, of course, we know that conception takes place about two weeks after the first day of the last period. So the actual time since conception (conceptual age) is two weeks less than the gestational age. But doctors still use gestational age when measuring the length of a pregnancy. The "due date" is assigned as forty weeks (approximately nine months) after the LMP, because the average term delivery occurs at this point in the pregnancy.
Trimesters
We speak of pregnancy as being divided into three segments of about three months each. As commonly defined, the first trimester begins with the date of the last menstrual period and runs for thirteen weeks and six days thereafter; the second trimester covers fourteen weeks' gestation to twenty-five weeks and six days; and the third trimester covers the period from there until birth.
Embryo
This term describes the baby from conception until about ten weeks' gestational age (eight weeks after conception). During the embryonic period, the baby transforms from a small collection of cells to an organism with identifiable features and internal organs and structures.
Fetus
This is what the baby is called from ten weeks' gestational age until birth. During the fetal period, the baby grows rapidly and the internal organs mature. Despite the name change, there is no abrupt change in the baby to mark the end of the embryonic period and the beginning of the fetal period. For this reason, we prefer to use the friendlier term baby throughout the entire pregnancy.