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OB Dating Atlas

OB Dating Atlas

Dr. Kiara Layne Blough (New York Presbyterian-Brooklyn Methodist Hospital), Dr. Sarah Jones (Kings County SUNY Downstate)

Editors: Dr. Brandi Ring, FACOG, FAWM, Dr. Matthew Riscinti, Denver Health Ultrasound

Introduction

Accurate determination of gestational age is a critical component of early pregnancy evaluation and can positively affect pregnancy outcomes. Accurate dating allows for appropriate timing of obstetric care, assessment of fetal growth and development, and even helps avoid unnecessary pregnancy and labor interventions[1,4]. Furthermore, in the acute care setting, fetal biometrics used to date pregnancies can provide critical insight into early detection of unhealthy pregnancies.

The first step of dating a pregnancy is determining the first day of the last menstrual period (LMP). However, variability in menstrual cycle length and timing of ovulation can lead to inaccurate dating. Additionally, approximately one-half of women accurately recall their LMP, or are simply unsure or uncertain [6]. When appropriately conducted, obstetric ultrasound in the first trimester is the most accurate way to date a pregnancy [1].

This project aims to provide a straightforward guide to obstetric dating in the outpatient and acute care settings. We describe which biometric markers to use based on trimester, their accuracy, and clinical pearls to guide you in approximating the gestational age of a pregnancy by ultrasound.

Fetal biometric markers used to assess gestational age and their accuracy based on trimester. [1,2,3]

OB Dating Images:

Featured
Week 4 - Mean Sac Diameter (MSD)
Week 4 - Mean Sac Diameter (MSD)

A 28-year-old G0 with an unknown last menstrual period (LMP) presents for a missed period and a positive urine pregnancy (UPT) test two days ago. On abdominal ultrasound, the mean sac diameter is .92 cm consistent with a 4w6d intrauterine pregnancy (IUP).

General Rules:

  • MSD is the earliest measurement that can assess GA.

  • MSD can be used until the embryo is present (5w0d-8w0d), then use the CRL.

  • Mean of 3 orthogonal measurements (length, width, and height) of fluid-filled space in sac.

  • Gestational Age (days) = MSD (mm) + 25

    • This equation can be used when using a portable or handheld device that does not calculate the gestational age automatically.

  • Accuracy3: ± 5-7 days

Pearls:

  • A gestational sac alone cannot confirm a pregnancy. In the presence of an ectopic pregnancy, a pseudogestational sac may be present.

  • An MSD > 12.0 cm with no yolk sac visualized should raise concerns for a missed abortion or blighted ovum and should be further investigated.

Week 7 - Mean Sac Diameter (MSD)
Week 7 - Mean Sac Diameter (MSD)

A 21-year-old G3P3 with an unknown LMP presents for a missed period and a positive UPT. On abdominal ultrasound, a yolk sac is present. Three measurements of the gestational sac are 2.1 cm, 2.7 cm, and 2.5 cm, giving a mean sac diameter of 2.4 cm, consistent with a 7w0d IUP.

General Rules:

  • MSD is the earliest measurement that can assess GA.

  • MSD can be used until the embryo is present (5w0d-8w0d), then use the CRL.

  • Mean of 3 orthogonal measurements (length, width, and height) of fluid-filled space in sac.

  • Gestational Age (days) = MSD (mm) + 25

    • This equation can be used when using a portable or handheld device that does not calculate the gestational age automatically.

  • Accuracy3: ± 5-7 days

Week 10 - Crown Rump Length (CRL)
Week 10 - Crown Rump Length (CRL)

A 24-year-old G0 with an unknown LMP presents for a missed period and confirmation of pregnancy. She had a positive UPT 5 days ago. On abdominal ultrasound, an embryo is visualized. The crown-rump length measures 3.84 cm consistent with a 10w5d IUP.

General Rules:

  • Most accurate single measurement once the embryo is present (6w0d-13w6d) [1]

  • Mean of 3 linear measurements from the outer margin of the cranium/cephalic pole to caudal rump in the true midsagittal plane.

  • Gestational Age (weeks) = CRL (cm) + 6.5

    • This equation can be used when using a portable or handheld device that does not auto calculate the gestational age.

  • Accuracy3: ± 5 - 7 days

Week 15 - Biparietal Diameter (BPD)
Week 15 - Biparietal Diameter (BPD)

A 26-year-old G4P3 with an unknown LMP and no prior prenatal care presents with vaginal spotting. She had a positive home UPT 3 weeks. On abdominal ultrasound, the biparietal diameter measures 2.79 cm consistent with a 15w0d IUP.

General Rules:

  • Used to estimate gestational age starting at 14w0d

  • Both measurements should be taken in the transverse plane at the level of the of thalami and cavum septum pellucidum with no cerebellum in view

  • BPD: measure from the outer edge of parietal bone to inner edge on the opposite side

  • Accuracy3: ± 7 - 12 d

Week 17 - Femur Length
Week 17 - Femur Length

A 34-year-old G6P5 with an LMP of “a few months ago” presents for vaginal spotting. She had a positive UPT 5 days ago. FL measures 2.42cm, consistent with a 17w2d IUP.

General Rules:

  • Measure diaphyseal length only (excluding epiphysis) with the length of the bone perpendicular to the ultrasound beam

  • A simple measure to get in a critically ill medical or surgical patient to determine if the fetus is viable outside of the uterus [4]

  • Accuracy3: ± 7 - 17 d

Week 22 - Biparietal Diameter (BPD)
Week 22 - Biparietal Diameter (BPD)

An 18-year-old G1P0 with an LMP of “a few months ago” and no prior prenatal care presents to initiate care. She had a positive home UPT two months ago. On abdominal ultrasound, there is positive cardiac activity and positive fetal movement. The biparietal diameter measures 5.4 cm consistent with a 22w0d IUP.

General Rules:

  • Used to estimate gestational age starting at 14w0d

  • Both measurements should be taken in the transverse plane at the level of the of thalami and cavum septum pellucidum with no cerebellum in view

  • BPD: measure from the outer edge (closest to the probe) of parietal bone to inner edge on the opposite side (farthest away from u/s probe)

  • Accuracy3: ± 7 - 12 d

Week 25 - Abdominal Circumference (AC)
Week 25 - Abdominal Circumference (AC)

A 19-year-old G0 with an unknown LMP presents for a missed period and light vaginal bleeding. UPT is positive and on abdominal ultrasound, abdominal circumference measures 21.4 cm confirming a 25w6d IUP.

General Rules:

  • Measure the in the transverse section around the skin line making sure to be in the same plane as the stomach, umbilical vein, and portal sinus

  • It is important to take multiple measurements here, as measurements can be altered due to fetal breathing motion.

  • Accuracy [3]: ± 7-17 days

References:

  1. Pettker CM, Goldberg JD, El-Sayed YY, Copel JA. Committee Opinion 700: Method for Estimating the Due Date. Committee on Obstetric Practice, the American Institute of Ultrasound in Medicine, and the Society for Maternal-Fetal Medicine. Obstet Gynecol 2017 May;129(5):e150-e154. 

  2. Salomon LJ, Alfirevic Z, Bilardo CM, Chalouhi GE, Ghi T, Kagan KO, Lau TK, Papageorghiou AT, Raine-Fenning NJ, Stirnemann J, Suresh S, Tabor A, Timor-Tritsch IE, Toi A, Yeo G. ISUOG Practice Guidelines: performance of first trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2013 Jan;41(1):102-13.

  3. Verburg BO, Steegers EA, De Ridder M, et al. New charts for ultrasound dating of pregnancy and assessment of fetal growth: longitudinal data from a population-based cohort study. Ultrasound Obstet Gynecol 2008; 31:388.

  4. Bennett KA, Crane JM, O’shae P, Lacelle J, Hutchens D, Copel JA. First trimester ultrasound screening is effective in reducing postterm labor induction rates: a randomized controlled trial. Am J Obstet Gynecol 2004; 190:1077-81.