Obstetric Ultrasound: Pregnancy Assessment

Standard Ultrasound Index

• Lists key topics covered in a standard ultrasound procedure manual, including early pregnancy detection, gestational sac measurements, yolk sac and amnion assessment, crown rump length (CRL), and practice rules for early pregnancy.
• Outlines terminology, pain and loss in early pregnancy, guidelines for intrauterine pregnancy failure, ectopic pregnancy, and measurements for the 2nd and 3rd trimesters.
• Covers pre-birth checks, including positioning, placenta, and umbilical cord assessments.

Gestation Weeks 4-10

• Normal findings include the gestational sac (GS), yolk sac (YS), and embryo (fetal pole).
• Accurate Mean Sac Diameter (MSD) and Crown Rump Length (CRL) measurements are essential.
• Assessing viability and understanding terminology for non-viable pregnancies is important.
• Ectopic pregnancies, pregnancy of unknown location (PUL), hCG roles, molar pregnancy must be recognized.

Gestational Sac (6+ Weeks)

• Differentiate between gestational age and fetal pole and be aware of a difference of +- 2 weeks.
• Gestational age is calculated from the last menstruation, typically 2 weeks before fertilization, while embryonic age is from fertilization.
• Look for a small, round fluid collection inside the uterine cavity, normally positioned in mid-to-upper uterus, surrounded by a hyperechogenic rim.
• A gestational sac confirms intrauterine pregnancy with 100% positive predictive value (PPV).

Gestational Sac (8 Weeks)

• Observe the sac, embryo, yolk sac, and amnion.
• Detect heartbeat via transabdominal scan
• Structure is surrounded by a hyperechogenic rim and has an echogenic periphery.
• Intrauterine pregnancy is confirmed

Gestational Sac Location

• Find it in the upper half of the uterus
• The sac appears round or elliptical in longitudinal and transverse views, enclosed by a white echogenic rim (choriodecidual reaction).
• It's located in the uterine fundus.

Gestational Sac Measurement

• Find the sac is at the uterine fundus
• The diameter starts at 2 mm, growing to 5-6 mm by 5 weeks, increasing by about 1 mm daily in the first trimester.
• Measure from edge to edge
• Assess its placement, ensuring it's in the uterus, not the fallopian tubes (relevant in ectopic pregnancy).

Yolk Sac

• Visible from approximately 5 to 7 weeks
• MSD measures 5-6 mm.
• Typically seen 3-5 days before the embryo
• Diameter peaks at 6 mm at 10 weeks then decreases
• Usually not visible after the first trimester
• The number of yolk sacs usually equals the number of amnions (amniotic membrane).

Amnion

• A thin membrane that creates a closed sac around the embryo or fetus, containing watery fluid.
• First seen at 5.5 weeks, it’s a continuous, small membranous structure with the embryo, containing clear fluid.
• Separates the embryo and amniotic space from the extraembryonic coelom.
• It obliterates the coelomic cavity by 12-16 weeks.

Crown Rump Length (CRL)

• Measures the baby from crown to rump in centimeters
• It is measured from the top of the head (crown) to the bottom of the buttocks (rump) which estimates gestational age.
• Follow ISUOG guidelines, taking measurements from the midline sagittal section of the whole fetus.
• Ensure ideal horizontal orientation, maximized magnification, neutral position, amniotic fluid between chin and chest, with clearly defined endpoints.
• Limbs and yolk sac aren't included and can be measured from 6-7 weeks up to 14 weeks.

Early Pregnancy Practice Rules

• Gestational age of 5 weeks measures a GS of 10mm.
• Gestational age of 6 weeks measures a YS of 3mm.
• Heartbeat at 6.4 weeks is 110 bpm
• Gestational age of 6.3 weeks measures a CRL of 6mm
• Movement detectable at 7 weeks.

Early Pregnancy Terminology

• Viable pregnancies result in a liveborn baby.
• Non-viable pregnancies cannot result in a liveborn baby, including failed intrauterine or ectopic pregnancies.
• An intrauterine pregnancy with uncertain viability uses TV ultrasound to look for a gestational sac with no heartbeat.
• An empty sac shows no structures or minimal debris.
• Human Chorionic Gonadotropin levels should be positive, with serum hCG >5IU/mL.

Pain and Loss in Early Pregnancy

• Pain and vaginal bleeding occur in 1:5 pregnant women.
• With blood loss, 50% of pregnancies continue normally.
• If 50% of blood loss remains, it is considered non-viable with 10-15% being ectopic pregnancy.
• Pain in early pregnancy is a late symptom.
• Obstetric causes of pain include miscarriage, ectopic pregnancy, hemorrhage, ruptured corpus luteum cyst, and ovarian torsion.
• Non-obstetric causes include cystitis, appendicitis, and ureteric stones, and constipation.

Guidelines for Intrauterine Pregnancy Failure/Uncertain Viability

• Diagnostic criteria for pregnancy failure include CRL >7mm with no heartbeat and Mean Sac <25mm with no embryo.
• Absence of embryo with hb >2 weeks after scan GS without YS or Absence embryo with heartbeat >11 days after scan GS with YS suggests pregnancy failure.
• Suspicious findings include CRL <7mm with no heartbeat and Mean GS <16mm with no embryo.
• Absence of an embryo with heartbeat >7-13 days after scan GS without embryo and Absence embryo >6 weeks after LMP suggests further testing is needed.
• Other findings include an empty amnion adjacent to YS with no embryo and a small GS in relation to embryo size (

Key topics in obstetric ultrasound include early pregnancy detection, gestational sac measurements, and fetal viability assessment. Detailed guidelines cover intrauterine pregnancy failure, ectopic pregnancy, and measurements for the 2nd and 3rd trimesters. Also covers pre-birth checks.