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Antenatal Care: Establishing Gestational Age with Ultrasound

by Veronica Gillispie, MD, FACOG
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    So moving on to that ultrasound and establishing the gestational age. Again using ultrasound the 1st trimester is the most accurate. How we use it? We measure the crown-rump length. So the distance from the crown of the fetus to the rump. We expect that to be within one week of the last menstrual period. So let me explain that in a little bit detail. If by a patient's last menstrual period, the fetus is measuring 8 weeks and 2 days. And by our ultrasound, the measurement is 6 weeks and 5 days. We would refer to the ultrasound. Last menstrual period again is only accurate if it's a 28 day cycle and it's regular. The ultrasound is only going to tell us how far along the pregnancy is based on the date of conception. So in that case we will go with the gestational age the ultrasound is giving us. Let me give you another example. If the last menstrual period says the patient is 7 weeks and 2 days, and our ultrasound says she's 7 weeks and 5 days, which one would we choose. You guessed it right. We would use the last menstrual period. Because the last menstrual period in the ultrasound are within one week of each other. Now, in the 2nd and 3rd trimester, we can still use that for establishing gestational age. But it is not quite as accurate as we do in the 1st trimester. In the 2nd and 3rd trimester we use the biparietal diameter which is the distance between the parietal bones in the head. We use the abdominal circumference. And we use the femur length to determine the gestational age. Now here is a little rule. In the first trimester, the ultrasound can be off by one week. 2nd trimester...

    About the Lecture

    The lecture Antenatal Care: Establishing Gestational Age with Ultrasound by Veronica Gillispie, MD, FACOG is from the course Antenatal Care.


    Included Quiz Questions

    1. The gestational age calculated by transvaginal ultrasound should be used to calculate the estimated due date.
    2. The gestational age based off her last menstrual period should be used to calculate the estimated due date.
    3. An average gestational age should be calculated based off her last menstrual period and the transvaginal ultrasound.
    4. Neither gestational age should be used. A more accurate dating scan should be done in the second trimester.
    5. A repeat transvaginal ultrasound must be done later in the first trimester to determine an accurate gestational age.
    1. Second trimester ultrasounds are most accurate for establishing gestational age.
    2. First trimester ultrasounds can estimate the gestational age within one week of accuracy.
    3. Second trimester ultrasounds can estimate the gestational age within two weeks of accuracy.
    4. Third trimester ultrasounds can estimate the gestational age within threes weeks of accuracy.
    5. If a first trimester ultrasound measures a gestational age within one week of the last menstrual period, the last menstrual period should be used to establish the gestational age.
    1. 8 weeks 0 days
    2. 7 weeks 5 days
    3. 7 weeks 6 days
    4. Unable to determine given inconsistency of LMP and first trimester ultrasound
    5. 8 weeks 5 days.
    1. A first trimester measure of the crown-rump length on transvaginal ultrasound
    2. A first trimester measure of the biparietal diameter on transvaginal ultrasound
    3. A second trimester measure of the biparietal diameter, abdominal circumference, and femur length on transabdominal ultrasound
    4. A serum human chorionic gonadtropin level
    5. A second trimester crown-rump length on transabdominal ultrasound
    1. Fetal, maternal, and functional placental factors.
    2. Maternal and functional placental factors only
    3. Fetal factors only
    4. Date of conception only
    5. Ultrasound technique variability only
    1. Biparietal diameter
    2. Chest circumference
    3. Tibia length
    4. Placental thickness
    5. Crown-rump length

    Author of lecture Antenatal Care: Establishing Gestational Age with Ultrasound

     Veronica Gillispie, MD, FACOG

    Veronica Gillispie, MD, FACOG


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    it is very basic
    By Kalpana N. on 23. January 2017 for Antenatal Care: Establishing Gestational Age with Ultrasound

    it is very basic, not in detailed lecture,try to tell topic in detailed manner