pregnancy
Meaning of pregnancy: Counted in weeks from a date that precedes conception, so the number in a scan report runs about two weeks ahead of the biology.
Definition of pregnancy
Pregnancy is the state that follows fertilization of an egg by a sperm, once the fertilized egg has implanted in the lining of the uterus and begun to develop. Over the weeks that follow, the placenta forms and supplies oxygen and nutrients while removing waste, and the body adapts extensively to support the developing embryo and later fetus.
How is a pregnancy dated and divided?
Weeks are counted from the first day of the last menstrual period, not from conception, because that date is one a person is far more likely to know. Fertilization occurs roughly two weeks into that count, so gestational age runs about two weeks ahead of the age of the pregnancy itself. An early ultrasound measurement of the embryo is more accurate than the date when cycles are irregular, and the dating is adjusted accordingly. The span is grouped into three trimesters, each with its own developmental pattern and its own focus in prenatal care.
What develops when?
| Stage | What is happening |
|---|---|
| First trimester | Implantation, formation of the placenta, and the period in which the embryo lays down every major organ system |
| Second trimester | The fetus grows rapidly, movement is felt, and detailed anatomy scanning is usual |
| Third trimester | Weight gain, lung maturation, and positioning for birth |
The switch in name from embryo to fetus happens at the end of the eighth week after fertilization, once the basic structures are formed. Because organ formation is concentrated in the earliest weeks, that period is the most sensitive to anything that interferes with development, often before a pregnancy has been recognized.
How does the body change?
Blood volume rises substantially, the heart pumps more each minute, and the kidneys filter faster, which shifts the normal range of many laboratory results so that values are read against pregnancy-specific expectations rather than usual ones. Hormones loosen ligaments, slow the movement of the gut and alter how the body handles glucose. Routine care follows growth and wellbeing through examination, blood and urine testing and ultrasound.
How can a pregnancy end?
Most end with the birth of a baby at or near full term. A pregnancy can also end early through miscarriage, which is common in the first trimester, or be interrupted for medical or personal reasons. An ectopic pregnancy, in which implantation occurs outside the uterus, cannot continue and is a serious emergency because of the risk of internal bleeding.
Also known as
Used in a sentence
Routine ultrasound was performed to confirm the gestational age of the pregnancy.
Good to know
Gestational age is counted from the first day of the last menstrual period rather than from conception, which places fertilization at roughly two weeks into the count and makes the two figures differ throughout.