Tests During Pregnancy: Abdominal Ultrasound

Written by the Healthline Editorial Team | Published on March 15, 2012
Medically Reviewed by Joan K. Lingen, MD

Prenatal Visits

If you are a healthy woman with a normal pregnancy, your prenatal visits will usually be scheduled every month until 32 to 34 weeks, then every two weeks until 36 weeks, and then weekly until delivery. Obviously, this schedule is flexible. If you experience any complications between your scheduled visits, call your doctor immediately-a month is a long time to worry about something.

Will I Need a First Trimester Ultrasound?

Ultrasound is an essential tool for evaluating your baby during pregnancy. More than one half of all pregnant women in the United States undergo at least one ultrasound examination. Whether or not you receive an ultrasound during your first trimester of pregnancy depends on a number of factors related to your pregnancy, such as your risk for complications.

Common reasons for obtaining an ultrasound examination in the first trimester are to confirm fetal viability (that the fetus is alive) or to determine gestational age. Ultrasound determination of gestational age is helpful if your LMP (last menstrual period) is uncertain, you have a history of irregular periods, conception occurred during oral contraceptive use, or 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 complications, you have a history of regular periods, you are certain of the date your last menstrual period (LMP) began, and you receive prenatal care during your first trimester.

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 (a probe inserted into the vagina) is another option.

What Will a First Trimester Ultrasound Show?

Typically, a first trimester, endovaginal ultrasound reveals a gestational sac (the sac of water containing the fetus), a fetal pole (arms and legs developed to variable extents, depending on gestational age), and a yolk sac (a structure that provides nourishment to the fetus while the placenta is developing).

By approximately six weeks gestation, a fetal heartbeat is visible through an ultrasound, as well as multiple fetuses (twins, triplets, etc.). Evaluation of fetal anatomy is extremely limited in the first trimester. It is largely limited to assessing the presence or absence of major structures and the assessment of the relative sizes of the present structures.

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 (usually four weeks or less) or a blighted ovum (a fetus that does not develop).

An empty sac in the uterus may occur with an ectopic pregnancy (a pregnancy that implants somewhere other than the uterus). The most common site of an ectopic pregnancy is the fallopian tube. An ectopic pregnancy is a potentially life-threatening situation, due to the risk of hemorrhage. The presence or absence of an ectopic pregnancy can be further determined by checking for an appropriate rise in the amount of the hormone beta-hCG in the blood. A doubling of the level of beta-hCG over a period of approximately 48 hours is considered normal and virtually excludes the diagnosis of ectopic pregnancy.

What if There Is No Heartbeat?

A heartbeat may not be visible during an ultrasound if the examination is performed early in pregnancy-prior to the development of cardiac activity. In this situation, your doctor will repeat the ultrasound later in your pregnancy. Alternatively, the absence of cardiac activity may indicate that the fetus did not survive. Evaluation of surrounding structures, such as the size and shape of the gestational sac relative to the size of the fetus, can provide further clues to help differentiate fetal death from an otherwise normal but early gestation. 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 correlates with 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 establishing 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 and, hence, may not be a reliable indicator of conception date and of gestational age.

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