Your prenatal visits will probably
be scheduled every month until 32 to 34 weeks. After that, they will be every
two weeks until 36 weeks, and then weekly until delivery. This schedule is
flexible, depending on your pregnancy. If you experience any complications
between your scheduled visits, call your doctor immediately.
Will I Need a First Trimester
Ultrasound is an essential tool for evaluating your baby during pregnancy.
Whether or not you receive an ultrasound during your first trimester of
pregnancy depends on a number of factors related to your pregnancy, including
your risk for complications.
Common reasons for obtaining an ultrasound
examination in the first trimester are to confirm that the fetus
is alive (fetal viability) or to determine
gestational age. Ultrasound determination of gestational age is helpful if:
last menstrual period (LMP) is uncertain
have a history of irregular periods
occurred during oral contraceptive use
- if your
initial pelvic examination suggests a gestational age different from that
indicated by your LMP
You may not need an
ultrasound if you:
- have no risk factors for pregnancy
- you have a history of regular periods
- you are certain of the date your last
menstrual period (LMP) began
- you receive prenatal care during your first
What Happens During the Ultrasound?
Most ultrasound studies obtain an image by
sliding a transducer over the abdomen. The small size
of the fetus during the first trimester often necessitates higher resolution
than can be achieved from a vaginal approach. Endovaginal ultrasound
examination is another option. This is when a probe is inserted into the
What Will a First Trimester Ultrasound Show?
A first trimester endovaginal ultrasound typically
reveals three things. A gestational sac is the sac of water containing the
fetus. A fetal pole means that the arms and legs developed to variable extents, depending
on gestational age. A yolk sac is a structure
that provides nourishment to the fetus while the placenta is developing.
By about six weeks, a fetal
heartbeat is noted, as well as multiple fetuses (twins, triplets, etc.).
Evaluation of the anatomy is extremely limited in the first trimester.
What If the Ultrasound Shows a Sac Without
a Fetal Pole?
The presence of a sac without a fetal pole
usually indicates the presence of either an extremely early pregnancy, or a fetus that has not developed (blighted ovum).
An empty sac in the uterus may occur with a pregnancy that implants somewhere other than the uterus
(ectopic pregnancy). The most common site of an ectopic
pregnancy is the fallopian tube. This is a potentially life-threatening
situation, due to the risk of hemorrhage. Whether or not it’s an ectopic
pregnancy can be further determined by checking for a rise in the amount of the
hormone beta-hCG in the blood. A doubling of the level of beta-hCG over a
period of about 48 hours is considered normal and normally excludes the
diagnosis of ectopic pregnancy.
What If There Isn’t a Heartbeat?
A heartbeat may not be
visible during an ultrasound if the examination is performed early in pregnancy.
This would be prior to the development of cardiac activity. In this situation,
your doctor will repeat the ultrasound later in your pregnancy. The absence of
cardiac activity may also indicate that the fetus is not developing and may not
Furthermore, checking blood
levels of beta-hCG can help to distinguish between fetal death in the first
trimester and a normally developing, early pregnancy.
How Can Ultrasound Determine Gestational Age?
Usually, determining your baby's gestational
age and your due date is calculated from the
first day of your last menstrual period (LMP). If your last menstrual period is
unknown, an ultrasound evaluation of fetal size can determine gestational age
and due date.
Ultrasound determination of gestational age
is most accurate during the first trimester because there is little biological
variability in size from one pregnancy to the next and few factors that
significantly affect fetal growth.
Measurement of the fetal pole from one end
to the other is called the crown-rump length (CRL).
This measurement relates to the actual gestational age within five to seven
days. Typically, if the due date suggested by the CRL falls within about five
days of menstrual dating, the due date established by the LMP is kept
throughout pregnancy. If the due date suggested by the CRL falls outside this
range, the due date from the ultrasound is usually kept.
Ultrasound is particularly helpful in figuring
out the gestational age and the due date in women who are uncertain of their
last menstrual period, who have a history of irregular menses, or who conceived
while taking birth control pills. In the situation of irregular bleeding or conception on birth control pills, the
last episode of vaginal bleeding may not be associated with the actual LMP.
That would mean that LMP might not be a reliable indicator of conception date
and of gestational age.