Obstetric Ultrasound

• First 13 weeks of pregnancy following the last normal menstrual period

1. Conceptus phase: 3-5 weeks
   • Conception usually occurs ~2 weeks after a woman's last menstrual period (LMP) and the gestational sac is usually too small to be seen for ~3 weeks
2. Embryonic phase: 6-10 weeks
3. fetal phase: 10-12 weeks

• Confirming intrauterine pregnancy (IUP)
  • Early features supportive of an intrauterine pregnancy
    • Double decidual sac sign
    • Intradecidual sac sign (sac within decidua)
    • Double bleb sign (yolk and amniotic sacs inside chorionic cavity) ~5 1/2 weeks
• Dating of the pregnancy
  • Crown rump length (CRL) measurement is most accurate, however if no fetal pole is seen, the mean sac diameter (MSD) may be used.
• Assessment of suspected early pregnancy failure
• assessment of suspected ectopic pregnancy
• antenatal screening for aneuploidy (nuchal lucency measurement)

• CRL of ≥7mm and no heartbeat on a transvaginal scan
• MSD of ≥25mm and no embryo on a transvaginal scan
• Absence of embryo with heartbeat ≥2 weeks after a scan that showed a gestational sac without a yolk sac
• Absence of embryo with heartbeat ≥11 days after a scan that showed a gestational sac with a yolk sac

• CRL of <7mm and no heartbeat
• MSD of 16-24 mm and no embryo
• Absence of embryo with heartbeat 7-13 days after a scan that showed a gestational sac without a yolk sac
• Absence of embryo with heartbeat 7-10 days after a scan that showed a gestational sac with a yolk sac
• Absence of embryo ≥ 6 weeks after last menstrual period
• Amnion seen adjacent to yolk sac, with no visible embryo (empty amnion sign)
• Enlarged yolk sac (>7 mm)
• Small gestational sac in relation to the size of the embryo (<5 mm difference between mean sac diameter and crown-rump length)
• Large amniotic cavity (expanded amnion sign)

• Miscarriage
  • Threatened miscarriage (≤20 weeks presenting with spotting, mild abdominal pain and contractions, and a closed cervical os)
  • Missed miscarriage (a non-viable fetus within the uterus, without symptoms of a miscarriage)
  • Incomplete miscarriage (miscarriage where there are still retained products of conception within the uterus)
• Subchorionic haemorrhage
• Ectopic pregnancy
• Gestational trophoblastic disease
• Demise of a twin
• Implantation bleeding

• Placental abruption
• Placenta previa
• Vasa previa
• Gestational trophoblastic disease

• Low lying placenta

  Occurs when the placenta extends into the lower uterine segment and its edge lies too close to the internal os of the cervix, without covering it. The term is usually applied when the placental edge is within 0.5-5.0 cm of the internal cervical os. Some alternatively give the term when the placental edge is within 2 cm from the internal cervical os.

• Placenta previa

  Placenta praevia is a term given to an abnormally low position of the placenta such that it lies close to, or covers the internal cervical os.

• Placenta accreta

  Condition seen at the milder end of the spectrum of abnormal placental adherence. In a placenta accreta, the placental villi extend beyond the confines of the endometrium and attach to the superficial aspect of the myometrium but without deep invasion.

• Placenta increta

  An intermediate level in the spectrum of abnormal placental villous implantation and accounts for ~20% of such cases. The placental villi extend beyond the confines of the endometrium and invades the myometrium.

• Placenta percreta

  The most severe but least common form of the spectrum of abnormal placental villous adherence, where there is a transmural extension of placental tissue across the myometrium with serosal breach. It carries severe maternal as well as fetal risks.

• Placental abruption

  Placental abruption refers to a premature separation of the normally implanted placenta after the 20th week of gestation and before the 3rd stage of labour. It is a potentially fatal complication of pregnancy and is a significant cause of third-trimester bleeding/antepartum haemorrhage.

  • Risk Factors:
    • Maternal trauma<
    • Age 35 years or older
    • Cigarette smoking
    • Cocaine use
    • Thrombophilia
    • Previous placental abruption
    • Chorioamnionitis
    • Prolonged rupture of membranes
    • Pre-eclampsia and maternal hypertension: often seen in as many as 50% of cases
    • Short umbilical cord
    • Increased parity
• Circumvallate placenta

  A variation in placental morphology in which, as a result of a small chorionic plate, the amnion and chorion fetal membranes ‘double back’ around the edge of the placenta.

  • Higher incidence of placental abruption
  • Increased risk of IUGR
• Marginal Cord Insertion
  • Insertion of cord into placental margin instead of near center of placenta
  • Associations:
    • Monochorionic twins pregnancy
    • IUGR
    • Preterm birth
    • Some authors state that there is no increased risk
• Vasa Previa
  • type I (present in ~ 90% of cases with vasa praevia ³): abnormal fetal vessels connect a velamentous cord insertion with the main body of the placenta
  • type II
    • abnormal vessels connect portions of a bilobed placenta
    • placenta with a succenturiate lobe: due to this association, vasa praevia needs to be excluded in patients with variant placental morphology
• Velamentous Cord Insertion
  • Insertion of the umbilical cord on the chorioamniotic membranes rather than on the placental mass
  • Associated with bilobed placenta, placenta previa, single umbilical artery, uterine anomalies, twin pregnancy
  • Complications:
    • Type I vasa previa
    • Increased risk of intrauterine growth restriction (IUGR)
    • Increased risk of growth discordance if in a twin pregnancy
    • Increased risk of twin to twin transfusion syndrome
• Succenturiate

  A variation in placental morphology and refers to a smaller accessory placental lobe that is separate to the main disc of the placenta. There can be more than one succenturiate lobe.

  • Increased incidence of type II Vasa previa
  • Increased incidence of postpartum haemorrhage due to retained placental tissue
• Bilobate

  Also referred as bipartite placenta, is a variation in placental morphology and refers to a placenta separated into two near equal-sized lobes. (If more than two lobes are present, it is termed a trilobed, four-lobed and so on.)

  • Associated with velamentous insertion of the cord
• Single umbilical artery

  The occurrence of a single umbilical artery is thought to be due to secondary atresia or atrophy rather than primary agenesis of the artery. The remaining single artery is often quite large often approaches the size of the umbilical vein (which in a normal situation is usually larger than the artery).

  • Recognised associations are thought to be present in ~35% (range 25-43%) of cases:
    • Lesser number of coils in umbilical cord
    • Umbilical arterial aneurysm
    • Twin reversed arterial perfusion sequence
    • Increased incidence of intra-uterine growth retardation (IUGR)
  • When found with other fetal anomalies, it can be also be associated with:
    • Chromosomal anomalies (trisomy 21: 50% had a SUA, trisomy 18: 25% had a SUA, trisomy 13: 7.5% had SUA)
    • Persistent right umbilical vein
    • Renal agenesis (usually occurs on the side the artery is absent)
    • Sirenomelia
    • Velamentous insertion of the cord

• 3 – 5 cm in length

• Polyhydramnios
  • 60% unknown cause
  • 20% fetal cause (eg. swallowing/obstruction, abd defects, atresias, dwarfs)
  • 20% maternal cause (Rh incompatibility, diabetes)
• Oligohydramnios AFI in 3rd trimester <8 cm is worrisome, < 5 cm is an emergency
  • DDx: DRIPPC
  • Demise
  • Renal
  • IUGR (and placental insufficiency)
  • PROM
  • Post-dates
  • Chromosomes (amnion fuses with chorion ~ 16 weeks, so try to do amniocentesis >16 wks because riskier before then)

• Intrauterine growth retardation (IUGR)
  • follow-up US interval of 3-4 wks to exclude differences attributed to normal variance
• Best dating parameters
  • First trimester: crown-rump length (best time to date by US)
  • Second trimester: BPD
  • Third trimester: BPD, but, if head low, then femur length
• Dating can be off by the following and be normal (a standard deviation)
  • First trimester: 1 week
  • Second trimester: 2 weeks
  • Third trimester: 3 weeks

• Changes for different gestational ages

• Very resistive waveform

• Omphalocele
  • Physiologic up to 12 weeks
• Gastroschisis

• Extrophy
  • Weakness in anterior muscle
• Keyhole
  • In males with posterior urethral valves

• 45 degrees, pointing left
• size = 1/3 size of the chest
• tricuspid valve 1mm higher than mitral valve
• no VSDs or ASDs

• Second structure to appear on US after GS
• Should be <6mm
• Involutes around 12-16 weeks
• Provides nutrients
• Primary site of hematopoiesis in early development
• Resorbed into the small intestine @ site of Meckel’s diverticulum

• Nuchal Translucency
  • A finding during a specific period in the late first trimester and early second trimester (11.3-13.6 weeks)
  • Should not be confused with nuchal thickness which is measured in the second trimester
  • Thought to be related to dilated lymphatic channels and is considered a nonspecific sign of more generalized fetal abnormality
  • Measurement of the nuchal lucency requires careful attention to technique
• Nuchal fold thickness
  • A parameter that is measured in a second trimester scan (at ~18-22 weeks)
  • The proposed etiology of increased nuchal thickness is as result of congenital heart disease and lymphatic obstruction
  • Measurement of the nuchal lucency requires careful attention to technique

• Diamniotic Dichorionic
  • 1 (fused) or 2 placentas
  • 10% mortality

• Diamniotic Monochorionic
  • 1 placenta
  • 25% mortality
  • risk of twin-twin transfusion syndrome
  • risk of stuck twin

• Monamniotic Monochorionic
  • 1 placenta
  • 50% mortality
  • risk of cord entanglement

• Telling amnion from the chorion:
  • Sex
  • Placenta
  • Membrane

• Normal pregancy
• Anembryonic pregnancy (sac but no pole)
• Embryonic/fetal demise

• Very early normal pregnancy
• Spontaneous abortion
  • ~30% of all pregnancies <12 wks
• Ectopic pregnancy
  • Beware of pseudogestational sac – endometrial hormonal reaction can mimmick IUP
  • Look in the adnexae for ectopic

• Uterus larger than pregnancy age
• Abnormally high β-hCG
• Hyperemesis
• Hypertension
• 50% have theca lutein cysts (multiseptated cystic ovarian masses) 2° to v high beta HCG levels
• Complete hydatidiform mole (CHM):
  • Commonest manifestation of GTD (up to 80%)
  • 46XX or 46XY: paternal chromosomes only
  • No fetus
  • β-hCG markedly elevated
  • Atypia of cells present
  • Treatment involves:
    • Curettage +/- hysterectomy in older women
    • Follow-up urinary β-hCG for 6-12 months
  • May progress to:
    • Invasive mole: ≈15%
    • Choriocarcinoma: ≈5%
• Partial hydatidiform mole (PHM):
  • 69XXX or 69XXY (paternal and maternal chromosomes)
  • May have a fetus or fetal components
  • β-hCG moderately elevated
  • No cellular atypia
• Invasive mole
  • Distorts uterine zonal structures
  • Boundaries between the tumour and myometrium are irregular and indistinct
  • May also invade parametrial tissue and blood vessels
• Gestational choriocarcinoma
  • May look identical to hydatidiform mole
  • Arises following known molar pregnancy (50%), miscarriage (30%), normal pregnancy (20%)
  • Can appear to have less vascularity than an invasive mole
  • Higher β-hCG levels even than a complete mole
  • Solid component with visualised invasion
  • Tends to invade myometrium through venous plexuses
    • Patients often can present with multiple metastases without an easily identified primary, as it can often be small in an otherwise normal placenta
  • Metastases can occur in:
    • Lungs - ~80%
    • Vagina - ~30%
    • Pelvis - 20% 
    • Liver and Brain - ~10%
• Placental site trophoblastic tumour (PSTT)
  • Rare form
  • Produces small amounts of β-hCG
• Epithelioid trophoblastic tumour (ETT)
  • Extremely rare form