Ultrasound in Obstetrics

Basics of Ultrasound in Obstetrics

As an obstetrician, getting ultrasounds done at various stages of pregnancy to gather different information is crucial. Ultrasound is a very important tool during pregnancy.

This video will cover:

Basics of ultrasound in obstetrics
First trimester ultrasound
Second trimester ultrasound
Third trimester ultrasound
Fetal biometry
Congenital anomalies (neural tube defects, nuchal translucencies)
Doppler ultrasound during pregnancy (middle cerebral artery, umbilical artery, uterine artery)

First Trimester Ultrasound

Need for Ultrasound

An ultrasound done in the first trimester provides a lot of information:

Fetal Viability: Determines whether the fetus is viable or not.
Pregnancy Location: Determines whether the pregnancy is intrauterine or ectopic.
Gestational Age: Provides an accurate estimation of gestational age.
Multi-fetal Pregnancy: Determines the number of fetuses and their chorionicity.
Aneuploidy Screening: Used for screening of aneuploidy.
Diagnosing Congenital Anomalies: Detects certain congenital anomalies, although the best time is the second trimester.
Chorionic Villi Sampling: Guides ultrasound-guided chorionic villi sampling.

Routine Ultrasound

Ultrasound is very safe in pregnancy. The maximum radiation exposure for a pregnant female is up to 5 rads. First trimester ultrasounds usually prefer a transvaginal scan over a transabdominal scan because structures are visible earlier. Structures are visible on transabdominal scans one week after they are visible on transvaginal sonography.

The frequency of a transvaginal sonography probe is $\geq 5$ MHz, whereas for a transabdominal scan, it is 3 to 3.5 MHz.

Modes of Ultrasound

The modes of ultrasound used in obstetrics are:

B mode (Brightness mode): Used for identifying intrauterine structures.
M mode (Motion mode): Used to detect cardiac activity.

Understanding Ultrasound Images

Black structures are hypoechoic or anechoic, indicating fluid (e.g., amniotic fluid or hydrocephalus).
Grey structures are isoechoic.
White structures are hyperechoic (e.g., bones and teeth).

Doppler ultrasound is generally avoided in very early pregnancy due to its thermal effects, as hyperthermia is a teratogen.

Intrauterine Pregnancy

The first structure seen on ultrasound when a female has a positive urine pregnancy test is the gestational sac. The blastocyst implants in the endometrium interstitially. The first sign on ultrasound is the intradecidual sign.

The gestational sac appears as a symmetrical fluid-filled cavity with an echogenic rim (chorionic rim).

The gestational sac can be seen earliest on a transvaginal scan between 4.5 to 5 weeks of pregnancy calculated from the first day of the last menstrual period.

A gestational sac confirms pregnancy but does not confirm whether it is an intrauterine pregnancy because a pseudo gestational sac can occur in ectopic pregnancies.

Clinical Vignette

A patient presents with her last menstrual period four weeks ago, denies symptoms, and has a history of ectopic pregnancy. The most appropriate action is to order a serum quantitative pregnancy test because, at less than 4 weeks 3 days, ultrasound is not as beneficial.

Discriminatory Score of hCG

The discriminatory score or 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

Serial assessment of HCG is essential because in early intrauterine pregnancies, HCG levels nearly double every 48 hours, whereas in ectopic pregnancy, the levels increase but do not double.

Mean Sac Diameter

In a gestational sac, the mean sac diameter is the mean of three perpendicular diameters and is used to determine gestational age in very early pregnancy.

A gestational sac confirms pregnancy but not the location. Two gestational sacs in the uterus suggest a dichorionic twin pregnancy. However, it is not diagnostic.

Yolk Sac

Yolk sac is the first structure to appear inside a gestational sac. If a yolk sac appears inside an intrauterine gestational sac, it confirms intrauterine pregnancy. If it appears in a gestational sac in the tubes, it confirms ectopic pregnancy.

A yolk sac can be seen on transvaginal sonography at five weeks, appearing like a single bleb inside the gestational sac.

Twin Pregnancy

In twin pregnancies:

Gestational sac is a marker of chorionicity.
Yolk sac is a marker of amnionicity.
Two gestational sacs and two yolk sacs suggest dichorionic, diamniotic twins.
A single gestational sac with two yolk sacs suggests monochorionic, diamniotic twins.

The best time for determining chorionicity is 10 to 14 weeks.

Fetal Pole

A fetal pole is seen on TVS between five weeks to five and a half weeks. When the fetal pole is visible, the crown rump length can be measured, which is the distance between the cephalic pole and the rump.

Fetal pole or yolk sac present inside a gestational sac in the tubes (with or without cardiac activity) confirms ectopic pregnancy.

Crown Rump Length

Crown rump length is measured from the cephalic end to the rump.

A critical cutoff to visualize fetal pole is when the gestational sac diameter is 25 millimeters. If the diameter of the gestational sac is more than 25 millimeters and a fetal pole is not visible, that's an abnormal pregnancy, called an embryonic pregnancy or blighted ovum.

Ultrasound Signs in Pregnancy

When a gestational sac is seen, interstitial implantation occurs, leading to the intradecidual sign which is a black fluid-filled cavity surrounded by an echogenic rim (chorionic rim or gestational sac).

Decidua

When the blastocyst implants in the endometrium, the endometrium is called the decidua, which is divided into three parts:

Decidua basalis (below the blastocyst)
Decidua capsularis (separating the blastocyst from the uterine cavity)
Decidua parietalis (rest of the decidua)

The decidua capsularis and decidua parietalis appear as two concentric rings, called the double decidual sac sign. The inner ring corresponds to the decidua capsularis, and the outer ring corresponds to the decidua parietalis.

A double decidual sac sign indicates intrauterine pregnancy, but the absence does not diagnose ectopic pregnancy.

Yolk sac is visible by five weeks and degrades by 10 to 12 weeks, appearing as a single bleb inside the gestational sac.

After the yolk sac, a fetal pole will be seen, leading to a double bleb sign. Blastocyst has an inner cell mass and trophoblast. The inner cell mass forms a bilaminar germ cell layer (epiblast and hypoblast), forming two cavities: amniotic cavity and yolk sac. The amniotic sac will contain the fetal pole.

A double bleb sign means the amniotic sac and the yolk sac.

True vs. Pseudo Gestational Sac

True gestational sac: Seen in intrauterine pregnancy due to blastocyst implantation and is eccentric in location.
Pseudo gestational sac: Seen in ectopic pregnancy due to hormonal changes thickening the decidua and is centrally located. A pseudo gestational sac will not grow, and yolk sac, double decidual sac sign, and double bleb sign will be absent.

Cardiac Activity

To determine viability, cardiac activity can be seen on M mode on TVS between five and a half weeks to six weeks.

Criteria for Non-Viability

Mean sac diameter > 25 millimeters without a fetal pole or crown rump length.
Crown rump length > 7 millimeters without cardiac activity.

Clinical Case Scenarios

Beta hCG levels are 500 international units, and gestational sac not seen on TVS: Next step is to repeat ultrasound after one week.
Beta hCG levels are 2,800 international units, gestational sac is 13 millimeters, yolk sac seen, but crown rump length not seen: Next step is to wait for a week and repeat ultrasound. Can also repeat beta HCG after 48 hours to assess the trend (doubling indicates intrauterine pregnancy, slower rise indicates ectopic pregnancy, decreasing indicates non-viable pregnancy).
Six weeks amenorrhea with bleeding PV, gestational sac is 18 millimeters, no yolk sac, no embryo: Wait for one week because an embryonic pregnancy can be confirmed only if the gestational sac is > 25 millimeters without any embryo.

Ectopic Pregnancy

Ultrasound Signs of Ectopic Pregnancy

Diagnostic: Gestational sac with yolk sac or fetal pole (+/- cardiac activity) in fallopian tube.
Suggestive: Empty uterus, complex adnexal mass, increased vascularity (ring of fire pattern).

Ring of fire pattern is suggestive but not diagnostic of ectopic pregnancy.

Gestational Age Estimation

First trimester ultrasounds are important for determining gestational age. Gestational age is clinically determined by adding nine months plus seven days to the first day of the last menstrual period. However, ultrasound is relied upon in cases of:

History of oral combined pills intake.
Lactation.
Irregular cycles.
Unknown last menstrual period.
Size of the uterus on examination different from the predicted LMP.

Ultrasound Parameters

First trimester: Crown rump length.
Second trimester: Biparietal diameter.
Third trimester: Femur length (single best parameter for questions, but clinically a combination of parameters is used).

Earlier ultrasounds in pregnancy are more accurate in determining gestational age. If multiple ultrasounds are available, the first trimester ultrasound is the most accurate.

In twin pregnancies, gestational age/EDD is based on the larger twin. Suboptimally dated pregnancy means no ultrasound was done by 22 weeks.

In pregnancies with uncertain LMP and late first ultrasounds, serial ultrasounds are done. Other features include ossification of the distal femoral epiphysis (suggests a gestational age of 32) and ossification of proximal tibia and humeral epiphysis (suggests a gestational age of 35). Note: the age here is "gestational age", not fetal age.

Fetal Biometric Parameters

Crown Rump Length

Best parameter for estimating gestational age in the first trimester.
Can be used up to 13 weeks + 6 days or when crown rump length is < 84 millimeters.
Smallest measurable crown rump length is 5 millimeters.
Definition: Longest straight line measurement from the outer margin of the cephalic pole to the rump.
Can be measured from seven weeks onwards.
Most accurate between seven weeks to nine weeks (variation of only $\pm 5$ days).

Calculation

Gestational age in days $=$ crown rump length in millimeters $+ 42$ .

Mean Sac Diameter

Used before seven weeks. Maximum up to seven weeks. Not a very good parameter.

Calculation

Gestational age in days $=$ mean sac diameter $+ 30$ .

Smallest measurable g sac diameter is two to three millimeters. G sac diameters are less accurate when a fetal pole is seen.

Multifetal Pregnancy

In multifetal pregnancies, ultrasound helps in:

Confirming the number of fetuses.
Determining gestational age (based on the larger twin)
Determining chorionicity.

Chorionicity

Twin pregnancies can be dizygotic or monozygotic.

Dizygotic: Two ova are fertilized by two sperms, forming two zygotes. Always dichorionic diamniotic. Sex of the baby could be same or different.
Monozygotic: Single ova fertilized by a single sperm, forming a single zygote that divides into two. Twins can be dichorionic or monochorionic. Always have the same sex.