How Pregnancy Happens at a Glance

Pregnancy begins when a fertilized egg implants in
the uterus.
Getting pregnant can happen in different ways.
Understanding your fertility pattern can help with
avoiding pregnancy as well as getting pregnant.
Understanding how pregnancy happens is important.
If you want to avoid pregnancy, it helps to know
when a woman is most likely to get pregnant, and
when she is not. If you are trying to become
pregnant, it is important to know when getting
pregnant is most likely and the different ways it can
happen.
Whether you are trying to avoid pregnancy, trying
to get pregnant, or are just curious, you may have
lots of questions. Here are some answers about how
pregnancy happens.
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
or alternative insemination , 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 of the vagina or
are absorbed by the woman’s body.
The joining of egg and sperm is called
fertilization. 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.
BOY OR GIRL?
Millions of sperm are released when a man
ejaculates. About half of them have a gene that
could produce a boy. The other half have one that
could produce a girl.
IMPLANTATION
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.
TWINS?
It is possible for the developing ball of cells to
split up until about the end of week four. If it
splits into two, for example, identical twins can
develop. It is also possible for two eggs to be
released at ovulation. Fraternal — not identical —
twins can develop if both eggs get fertilized by
sperm and implant in the uterus.
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.