Tests During Pregnancy: Abdominal Ultrasound

Written by the Healthline Editorial Team | Published on December 17, 2014
Medically Reviewed by Brenda B. Spriggs, MD, MPH, FACP on December 17, 2014

Prenatal Visits

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?

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:

  • your last menstrual period (LMP) is uncertain
  • you have a history of irregular periods
  • conception 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 complications
  • 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 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 is another option. This is when a probe is inserted into the vagina.

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 survive.

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.

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