Ultrasound in Obstetrics

Integration of OBS and Radiology

As an obstetrician, it is crucial to perform ultrasounds at various stages of pregnancy to gather different information. Ultrasound is a vital tool during pregnancy. This includes discussion of Doppler ultrasound and its role.

Overview of Topics

  1. Basics of ultrasound in obstetrics.
  2. First trimester ultrasound.
  3. Second trimester ultrasound.
  4. Third trimester ultrasound.
  5. Fetal biometry.
  6. Congenital anomalies (neural tube defects, nuchal translucencies).
  7. Doppler ultrasound during pregnancy (Doppler of middle cerebral artery, umbilical artery, and uterine artery).

First Trimester Ultrasound

Indications

First trimester ultrasounds provide significant information:

  1. Fetal Viability: Determines if the fetus is viable.
  2. Pregnancy Location: Differentiates between intrauterine and ectopic pregnancies.
  3. Gestational Age: Provides accurate estimation of gestational age.
  4. Multi-fetal Pregnancy: Identifies the number of fetuses and their chorionicity.
  5. Aneuploidy Screening: Used for screening of aneuploidy.
  6. Congenital Anomaly Diagnosis: Detects certain congenital anomalies, although the best time for detection is the second trimester.
  7. Chorionic Villi Sampling: Guidance for ultrasound-guided chorionic villi sampling.

Routine Ultrasound in Pregnancy

Safety

Ultrasound is considered safe during pregnancy. The maximum radiation exposure allowed for a pregnant female is up to 5 rads.

Types of Scans

  1. Transvaginal Scan: Preferred in the first trimester for earlier visibility of structures compared to transabdominal scans. Structures are visible one week earlier on transvaginal scan.
    • Frequency of transvaginal probe: ≥5\geq 5 MHz.
  2. Transabdominal Scan: Structures are visible later than with transvaginal scans.
    • Frequency: 3 to 3.5 MHz.

Modes of Ultrasound

  1. B Mode (Brightness Mode): Used for identifying intrauterine structures. Structures appearing black are hypoechoic or anechoic, indicating fluid (e.g., amniotic fluid, hydrocephalus).
    • Grey structures are isoechoic.
    • White structures are hyperechoic (e.g., bones and teeth).
  2. M Mode (Motion Mode): Used to detect cardiac activity.

Doppler Ultrasound

  • Avoided in early pregnancy due to thermal effects, which can cause hyperthermia, a teratogen (raising of maternal temperature).

Intrauterine Pregnancy

First Sign on Ultrasound

The first structure visualized is the gestational sac due to interstitial implantation of the blastocyst in the endometrium, leading to the intradecidual sign.

Gestational Sac

  • Appearance: symmetrical fluid-filled cavity with an echogenic (chorionic) rim.
  • Visualization: earliest on transvaginal scan between 4.5 to 5 weeks of pregnancy (4 weeks, 3 days to 5 weeks).
    • Pregnancy is calculated from the first day of the last menstrual period.

Significance

  • Confirms pregnancy but does not confirm the location (intrauterine vs. extrauterine).
  • In intrauterine pregnancy, a true gestational sac is observed, whereas in ectopic pregnancy, a pseudo gestational sac may appear.

Clinical Scenario: Determining Pregnancy Location

  • Problem: A patient with a positive urine pregnancy test at the time of the missed period (before 4 weeks 3 days) who is anxious about potential ectopic pregnancy.
  • Solution: Serial assessment of HCG levels rather than immediate ultrasound.

Discriminatory Score of hCG

The discriminatory score (critical value) of hCG is the level above which a gestational sac should be visible in 100% of intrauterine pregnancies.

  • Transvaginal scan: 2,000 IU
  • Transabdominal scan: 6,500 IU
HCG levels & ectopic pregnancy

If HCG levels are above 2,000 IU and no sac is visible on TVS, it suggests an ectopic pregnancy. Serial assessments are necessary because:

  • In early intrauterine pregnancies, HCG levels nearly double every 48 hours.
  • In ectopic pregnancies, HCG levels increase, but not at the same doubling rate.

Mean Sac Diameter (MSD)

Mean of three perpendicular diameters of the gestational sac.

  • Used to estimate gestational age in early pregnancy.

Gestational Sac Location

  • Gestational sac in the uterus: Could be a true sac (intrauterine pregnancy) or a pseudo sac (ectopic pregnancy).
  • Gestational sac in the tube: Suggestive, but not diagnostic, of ectopic pregnancy.
Twin Pregnancy

If two gestational sacs are observed in the uterus, indicative of dichorionic twins, although not diagnostic.

Yolk Sac

The yolk sac is the first structure to appear inside the gestational sac.

  • Significance: Confirms the location of the pregnancy.
    • Yolk sac inside a gestational sac in the uterus confirms intrauterine pregnancy.
    • Yolk sac inside a gestational sac in the tubes confirms ectopic pregnancy.
  • Appearance: Visible on transvaginal sonography at five weeks as a single bleb inside the gestational sac.
  • Marker of amnionicity in twin pregnancies. Two yolk sacs suggest diamniotic twins.
  • Best time for determining chorionicity is between 10 to 14 weeks.

Twin Pregnancy and Yolk Sacs

Dichorionic Diamniotic Twins:

Two gestational sacs and two yolk sacs.

Monochorionic Diamniotic Twins:

Single gestational sac with two yolk sacs.

Fetal Pole

The fetal pole is seen on TVS between 5 to 5.5 weeks (5 weeks, 3 days).

Crown Rump Length (CRL)

Measurement from the cephalic pole to the rump.

  • Significance: similar to yolk sac, confirms ectopic pregnancy if present in yolk sac inside gestational sac in tubes (with or without cardiac activity).

Critical Cutoff

Critical cut-off to visualize fetal pole determined via gestational sac diameter of 25 mm. If the gestational sac exceeds 25 mm and a fetal pole is not visible, it indicates an abnormal pregnancy (anembryonic pregnancy or blighted ovum).

Clinical Implications

  • If gestational sac diameter is less than 25 mm and fetal pole is not visible, it is not immediately considered an abnormal pregnancy.
  • Anembryonic Pregnancy/Blighted Ovum: Diagnosed when gestational sac measures ≥25\geq 25 mm without a visible fetal pole.

Ultrasound Signs

Intradecidual Sign

Black fluid filled cavity surrounded by an echogenic (chorionic) rim due to interstitial implantation of the blastocyst.

Decidua Layers

When blastocyst implants, the endometrium becomes decidua divided into three parts:

  1. Decidua Basalis: below the blastocyst.
  2. Decidua Capsularis: separating the blastocyst from the uterine cavity.
  3. Decidua Parietalis: the rest of the decidua.

Double Decidual Sac Sign

Decidua capsularis and decidua parietalis appear as two concentric rings on ultrasound (visible by 4.5 to 5 weeks).

  • Inner ring: decidua capsularis

  • Outer ring: decidua parietalis

  • Indicates intrauterine pregnancy if present, but absence is not diagnostic of ectopic pregnancy.

Yolk Sac Appearance

Yolk sac is visible by 5 weeks and degrades by 10-12 weeks. It appears as a single bleb inside the gestational sac.

Double Bleb Sign

Appears when fetal pole is seen.

  • Two cavities are present: amniotic cavity and yolk sac.
Composition of blastocyst

The blastocyst contains:

  • Inner cell mass (forms bilaminar germ cell layer: epiblast and hypoblast).
  • Trophoblast (outer layer of cells).

Amniotic cavity: cavity surrounded by epiblast.
Yolk sac: cavity surrounded by hypoblast.

Double decidual sac sign vs double bleb sign

Double decidual sac sign: decidua capsularis and decidua parietalis.
Double bleb sign: amniotic sac and yolk sac.

The amniotic sac contains the fetal pole.

Pseudo Gestational Sac

Pseudo gestational sac seen in ectopic pregnancy. It's a thickened decidua due to hormonal changes.

True Gestational Sac vs. Pseudo Gestational Sac:
  • Location:
    • True sac: eccentric (on one side).
    • Pseudo sac: centrally located.
  • Growth:
    • True sac: grows daily (1 mm).
    • Pseudo sac: does not grow.
  • Contents:
    • True sac: yolk sac, double decidual sac sign, double bleb sign.
    • Pseudo sac: these are absent.

Cardiac Activity

Seen on M-mode on TVS between 5.5 to 6 weeks, indicating fetal viability.

Criteria for Non-Viability

  1. Mean Sac Diameter >25\gt 25 mm without fetal pole/CRL: blighted ovum/anembryonic pregnancy.
  2. CRL >7\gt 7 mm without cardiac activity: missed abortion.

Clinical Cases and Next Steps

  1. HCG Level: 500 IU, gestational sac not seen on TVS: repeat ultrasound after one week.
  2. HCG Level: 2,800 IU, gestational sac (13 mm) seen, yolk sac seen, CRL not seen: repeat ultrasound in one week; serial hCG estimation can be done.
    • In early intrauterine pregnancy, HCG levels nearly double every 48 hours; in ectopic pregnancy, the increase is less than doubling.
  3. Patient at 6 weeks amenorrhea with bleeding PV, gestational sac (18 mm), no yolk sac, no embryo: wait one week to repeat the ultrasound.

Signs of Ectopic Pregnancy on Ultrasound

  • Diagnostic Sign: Gestational sac with yolk sac or fetal pole (+/- cardiac activity) in fallopian tube.
  • Suggestive Signs:
    • Empty uterus
    • Complex adnexal mass
    • Increased vascularity (ring of fire pattern; not diagnostic).

Vascularity

Increased vascularity in the tubes known as ring of fire pattern, which is suggestive but not diagnostic of ectopic pregnancy.

Gestational Age Estimation

Clinical Determination

To the first day of the last menstrual period, add 9 months plus 7 days to determine expected date of delivery.

When to Rely on Ultrasound:

  • History of oral combined pills intake.
  • Lactating patient.
  • Irregular cycles.
  • Last menstrual period not known.
  • Size of the uterus on examination is different from the predicted LMP.

Ultrasound Parameters

  • First Trimester: Crown rump length (CRL).
  • Second Trimester: Biparietal diameter (BPD).
  • Third Trimester: Femur length (FL) is the single best parameter, although a combination of parameters is clinically used.

Earlier ultrasounds are more accurate for gestational age estimation.

Special Circumstances

  • Twin Pregnancy: Base calculations on the larger twin.
  • Suboptimally Dated Pregnancy: No ultrasound done by 22 weeks implies suboptimally dated pregnancy; serial ultrasounds 3-4 weeks apart to rule out IUGR and confirm EDD.
  • Pregnancy with Uncertain LMP and Late First Ultrasounds: Serial ultrasounds; features like ossification of distal femoral epiphysis (gestational age of 32) or ossification of proximal tibia and humeral epiphysis (gestational age of 35).

Fetal Biometric Parameters

Crown Rump Length (CRL)

  • Used up to 13 weeks + 6 days or when CRL < 84 mm.
  • Smallest CRL that can be measured is 5 mm.
  • Definition: longest straight line measurement of embryo from outer margin of cephalic pole to the rump.
  • Measurable from 7 weeks onwards; most accurate between 7-9 weeks (variation of ±5\pm 5 days).
  • Between 9 weeks to 13 weeks + 6 days, the accuracy is around ±7\pm 7 days.
Calculation of Gestational Age:
  • Gestational age in days = CRL (in mm) + 42.

Mean Sac Diameter (MSD)

Best before 7 weeks; mean of three perpendicular diameters of the sac.

  • Gestational age in days = MSD + 30.
  • Smallest G-sac diameter that can be measured is 2-3 mm.
Role of Ultrasound in Multifetal Pregnancy:
  • Confirm the number of fetuses.
  • Determine gestational age (based on the larger twin).
  • Determine chorionicity.

Twin Pregnancy

  • Dizygotic: Two ova fertilized by two sperms, two zygotes formed; always dichorionic diamniotic; sex of babies may be same or different.
  • Monozygotic: Single ova fertilized by single sperm, single zygote divides into two; twins may be dichorionic or monochorionic (depending on timing of division); always same sex.