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.

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

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

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

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