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FOR THE LOVE OF POCUS
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)
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)
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)
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
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)
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)
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