Pregnancy begins when a fertilized egg implants in the uterus. Getting pregnant can happen in different ways. Understanding how pregnancy happens is important.
When Does Pregnancy Begin?
In order for a woman to become pregnant, she must release an egg from her ovary — ovulation. Next, the egg and sperm must meet and form a single cell — fertilization. Then pregnancy begins when and if the fertilized egg attaches to a woman’s uterus and begins to grow — implantation.
Menstruation
During the first two weeks of a woman’s menstrual cycle she has her period. This usually lasts 3–7 days. After that, hormones make eggs mature in her ovaries and the lining of her uterus thickens.
Ovulation
Ovulation happens about two weeks before a woman’s next period would take place. The egg enters a fallopian tube and starts moving toward the uterus.
Fertilization
After vaginal intercourse, several hundred sperm travel up through the uterus and into the fallopian tubes. An egg may be in one of the tubes. One sperm may fertilize the egg. The millions of other sperm seep out or absorbed by the woman’s body. Fertilization is joining of egg and sperm. It is most likely to occur from sexual intercourse that happens during the five days before the egg is released or on the day of ovulation.
Implementation
The fertilized egg moves down the fallopian tube and divides into more and more cells, forming a ball. The ball of cells reaches the uterus about 3–4 days after fertilization. The ball floats in the uterus for another 2–3 days. Pregnancy begins if the ball of cells attaches to the lining of the uterus. This is called implantation. It usually starts about six days after fertilization and takes about 3–4 days to be complete. The embryo will develop from cells on the inside of the ball. The placenta will develop from the cells on the outside of the ball. Up to half of all fertilized eggs never implant. They pass out of women’s bodies during menstruation.
Gestational Age
Pregnancy is measured using “gestational age.” Gestational age starts on the first day of a woman’s last menstrual period (LMP). Gestational age can be confusing. Most people think of pregnancy as lasting nine months. And it’s true that a woman is pregnant for about nine months, but because pregnancy is measured from a woman’s last menstrual period — about 3–4 weeks before she is actually pregnant — a full-term pregnancy usually totals about 40 weeks LMP — roughly 10 months. Many women do not remember the exact date of their last menstrual period — that’s OK. The surest way to tell gestational age early in pregnancy is with ultrasound.