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Crown to Rump Length Ultrasounds

A positive pregnancy test marks the beginning of the prenatal period, which culminates in a baby's birth. One of the first things your health-care provider will want to determine is the date the baby will arrive. Having an accurate estimate of the delivery date is essential to prevent premature birth and the adverse effects that accompany it. Accurate dating also makes it possible to monitor development to confirm that the fetus reaches key milestones at the expected times. Ultrasonography plays a critical role in determining this date.

  1. Methods

    • One way to determine how "old" a pregnancy is --- its gestational age --- is to count the number of days since the woman's last menstrual period. A more precise method is to measure the distance between the crown of the fetus's head and the promontory of the buttocks (the rump) by ultrasound imaging. Ultrasonography uses high-frequency sound waves to create a picture of the body. When the fetus is viewed by ultrasound, the technician can measure the crown-rump length and determine the gestational age within four or five days. For example, a crown-rump length of 18 mm equals a gestational age of just more than eight weeks.

    Timing

    • Crown-rump length is measured during the first trimester because there is less variability between fetuses at that time. In later trimesters, pregnancies progress at different rates, so a crown-rump length would be harder to link to a particular gestational age.

    Requirements

    • The ultrasound technician must measure crown-rump length when the fetus is in a resting position. In that position, there is a natural curve between the crown and the rump, and the fetus assumes its full length. If the fetus is flexed during measurement, this curve shortens or straightens, leading to inaccurate assessment.

    Tools

    • The technician views the fetus using abdominal imaging (transabdominal ultrasound) or through the vagina (transvaginal ultrasound) using a probe called a transducer. The transvaginal transducer usually produces greater resolution, but both the transabdominal and transvaginal views are adequate for measuring crown-rump length.

    Risks

    • Ultrasound assessment is non-invasive. That means it does not harm the pregnant woman or her fetus. Nevertheless, most health-care providers try to limit ultrasound assessment to essential tasks because the health effects of frequent imaging have not been definitively established.

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