1. Trimesters of Pregnancy
During the first 8 weeks of pregnancy, your baby is called an embryo. During the rest of your pregnancy, the developing baby is referred to as a fetus.
Your pregnancy is divided into three 13-week periods called trimesters.
The first trimester is the one of greatest change for the developing
baby. During this time, your baby grows from a collection of cells the
size of the head of a pin to a fetus the size of a grapefruit. Organs
begin to develop, and your baby begins to look like a baby.
At the beginning of the second trimester,
your baby weighs less than an ounce (25g) and is only about 4 inches
(11cm) long. By the beginning of the third
trimester, it is almost 16 inches (40cm) long and weighs more than 2
pounds (1kg). At delivery, baby will weigh close to 7½ pounds (3.4kg)
and be about 21½ inches (54cm) long.
The first trimester is the one of
greatest change for the developing baby. Very few, if any, structures
in the fetus are formed after 12 weeks of pregnancy. In fact, by the
time you miss a period, 80% of baby’s organ development has already
occurred. This means your baby has formed all of its major organ
systems by the end of the first trimester. However, these structures
continue to grow and to mature until your baby is born.
When is it determined that I will have a girl or a boy?
At the time of fertilization. Sperm are
either male or female—all eggs are female. If a male sperm (carrying a
Y chromosome) fertilizes the egg, you will have a boy. If a female
sperm (carrying an X chromosome) fertilizes the egg, you will have a
If a baby is born before 38 weeks of pregnancy, he or she is called a preterm baby. An infant born between the 38 and 40 weeks of pregnancy is called a term baby or a full-term infant. A baby delivered after 42 weeks of pregnancy is called a postterm baby.
2. Normal Fetal Development
The Baby’s Size
Sometimes women express concern about
giving birth to a large baby. Many factors affect how big your baby
will be. If you are in good health, have no medical problems, don’t
gain too much weight and take good care of yourself during pregnancy,
you’ll probably have an average-size baby. Although weight varies
greatly from baby to baby, the average baby at term weighs 7 to 7½
pounds (3.2 to 3.4kg).
Some maternal factors do affect the size
of a baby at birth, including hypertension and diabetes, which are more
common in older pregnant women. Hypertension during pregnancy can cause
intrauterine-growth restriction (IUGR), which results in smaller
babies. Diabetes can cause blood-sugar problems. Blood-sugar levels are
higher in those with gestational diabetes or mild diabetes that is not
under control. Diabetes exposes the baby to higher sugar levels,
resulting in a larger baby.
In cases of
insulin-dependent diabetes, the result may be a smaller baby. Women
with insulin-dependent diabetes may have circulation problems, which
can result in IUGR and decreased blood flow to the baby.
3. The Fetal Environment
Your baby is growing and developing
inside a complex system within your body. There are three major parts
to this system, and each relies on the other to work together as a
complete unit. Your baby’s first home consists of the placenta, the
umbilical cord and the amniotic sac. Together they provide nourishment,
warmth and protection while your baby matures and prepares to live on
its own outside your uterus.
4. The Placenta
The placenta is a soft, round or oval
organ that grows with your baby. At 10 weeks, it weighs about ½ ounce
(12g); by the time your baby is born, it weighs about 1½ pounds (680g).
When the early pregnancy implants in your
uterus, the placenta grows and sends blood vessels into the uterine
wall. These blood vessels carry nourishment and oxygen from your blood
to baby for your baby’s use. Baby’s waste products pass back into your
bloodstream through these vessels for disposal by your body.
We once believed the placenta acted as a
barrier to all outside substances, but we now know this is not the
case. In some instances, the placenta cannot keep your baby from being
exposed to substances you are exposed to or you ingest. We know
alcohol, most medications, other substances (such as nicotine) and many
vitamins, minerals and herbs cross the placenta to your baby. This is
one reason women should avoid some substances during pregnancy.
The placenta is important to your
pregnancy and remains so until the birth of your baby. At that time,
when your uterus begins to shrink after your baby is born, the placenta
detaches from it and is delivered on its own.