Early Pregnancy & Gestational Sac: Weeks 4-10

Early Pregnancy: Gestation 4-10 Weeks

• In early pregnancy ultrasounds look for a normal gestational sac (GS), yolk sac (YS), and embryo (fetal pole)
• Mean Sac Diameter (MSD) and Crown Rump Length (CRL) are assessed.
• Evaluate viability using specific criteria and terminology for non-viable pregnancies
• Ectopic pregnancies are recognised, using required principles for pregnancies of unknown location (PUL)
• Follow the role of hCG and management of PUL
• Look for molar pregnancies

Confirming Pregnancy: Gestational Sac 6 Weeks+

• Be aware of gestational sac and fetal pole differences of +- 2 weeks
• Gestational age is calculated from the last menstruation, typically 2 weeks before fertilization
• Embryonic age measures the actual age from fertilization
• Ultrasound observations include:
  • A small, round fluid collection inside the uterine cavity
  • Normal positioning in the mid-to-upper uterine cavity
  • Hyperechogenic rim
  • Visibility around 4 weeks gestation via trans-vaginal ultrasound
• Note gestational age vs. embryonic age
• The first visible structure within the gestational sac confirms intrauterine pregnancy with 100% PPV
• The sac will be spherical in shape, with an echogenic periphery and a sonolucent center
• A vitelline duct attaches the sac to the embryo

Gestational Sac (GS) at 8 Weeks

• An 8-week sac displays a visible sac, embryo, yolk sac, and amnion
• A heartbeat can be detected using a transabdominal scan
• The sac is surrounded by a hyperechogenic rim
• There is an echogenic periphery
• Intrauterine pregnancy is confirmed with 100% certainty
• Be aware of differences in gestational age vs. embryo age

Gestational Sac Location

• Gestational sacs are within the upper half of the uterus
• In longitudinal and transverse views, the sac appears round or elliptical
• White echogenic rim surrounds the sac which indicates choriodecidual reaction
• The sac is located in the uterine fundus

Gestational Sac Measurement

• It is a fluid-filled structure surrounds the embryo
• The sac is located in the uterine fundus
• The diameter starts at about 2 mm and grows to 5-6 mm by 5 weeks
• Throughout the 1st trimester, the mean gestational sac diameter increases by roughly 1 mm per day
• Measure from the sac edge
• Correct placement is important, ensure the sac is in the uterus and NOT in the fallopian tubes (seen in ectopic pregnancies)
• Note gestational age and embryo age difference

Yolk Sac

• You can see them from about 5-7 weeks with MSD being about 5-6 mm
• Are generally seen 3-5 days before the embryo
• The diameter maxes out at 6mm at 10 weeks, then declines
• Usually not visible after the first trimester
• Number of yolk sacs equals the number of amnions (amniotic membranes)

Amnion

• It is a thin membrane that creates a closed sac containing watery fluid around the embryo or fetus.
• First seen at 5.5 weeks
• Is a small, membranous structure that is one with the embryo, and contains a clear fluid
• Separates the embryo and amniotic space from the extraembryonic coelom
• Obliterates the coelomic cavity by 12-16 weeks

Early Pregnancy Measurements: Crown Rump Length (CRL)

• Baby is measured in centimeters from the crown to the rump
• CRL measures the length of embryos and fetuses from the top of the head to the bottom of the buttocks using ultrasound to estimate gestational age
• ISUOG guidelines must be followed
• It must be taken from the midline sagittal section of the whole fetus
• Ideal orientation is horizontally
• Magnification should fill most of the width of the screen
• Features in neutral position
• Should have amniotic fluid between the chin and chest
• Endpoints should be clearly defined
• Limbs and yolk sac are not included, and it can be done starting at six or seven weeks up to 14 weeks

Early Pregnancy: Practice Rules and Terminology

• Gestational sac, yolk sac, heartbeat and movement measurements with normal gestational age using abdominal ultrasound:
  • GS at 5 weeks measures 10mm
  • YS at 6 weeks measures 3mm
  • Heartbeat at 6.4 weeks is 110 bpm
  • CRL at 6.3 weeks is 6mm -Movement at 7 weeks
• Terminology includes:
  • Viable: Results in liveborn baby
  • Non-Viable: Cannot result in liveborn baby (failed intrauterine pregnancy, ectopic pregnancy)
  • Intrauterine Pregnancy (Uncertain viability): TV Ultrasound - Intrauterine GS with no Heartbeat
  • Empty Sac: Absent structures, minima debris, no heartbeat
  • Human Chorionic Gonadotropin: Positive serum pregnancy test serum hCG>5IU/mL

Pain and Loss in Early Pregnancy

• 1:5 pregnant women experience vaginal bleeding and pain
• 50% with bleeding will continue into a normal pregnancy
• 50% remaining exhibit blood loss, of which 10-15% are Ectopic pregnancies
• Pain in early pregnancy is a late symptom
• Causes can be:
  • Obstetric (Miscarriage, ectopic, hemorrhage ruptured corpus luteum cyst, ovarian torsion)
  • Non-obstetric (Cystitis, appendicitis, ureteric stones, constipation)

Guidelines for Intrauterine Pregnancy Failure and Uncertain Viability

• Sonographic diagnostic factors for pregnancy failure:
  • CRL >7mm with no heartbeat
  • Mean GS <25mm with no embryo
  • Absence of embryo with heartbeat >2 weeks after scan GS without YS
  • Absence of embryo with heartbeat >11 days after scan GS with YS
• Suspicious / not diagnostic of failure:
  • CRL <7mm with no heartbeat
  • Absence of embryo with heartbeat >7-13 days after scan GS without embryo
  • Absence embryo >6wks after LMP
  • Empty amnion adjacent to YS no embryo
  • Small GS in relation to size of embryo (

Examine early pregnancy ultrasounds, seeking a normal gestational sac, yolk sac, and embryo. Assess Mean Sac Diameter and Crown Rump Length. Recognize ectopic and molar pregnancies and follow the role of hCG in managing pregnancies of unknown location.