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Obstetrics - USG

Scan Planes:

Whole abdominal region is scanned using various movements of probe.

3.5 to 5 MHz probe
7.0 to 8.0 MHz probe is used for transvaginal ultrasound.

Transvaginal imaging can utilize higher probe frequencies because the area of interest, e.g. the ovary is much closer to the transducer – than with a trans abdominal probe.

The patient should be asked to come with full bladder for trans abdominal scan and should completely void her bladder for trans vaginal ultrasound.

Position is Supine for abdominal and lithotomy position for transvaginal scans.

Fetal age estimation

First trimester- Yolk sac, CRL, Gestational sac.
Second trimester- BPD, HC, AC, FL.
Third trimester- Femur Length
Neonatal- Trans-fonatnelle ultrasonography

First Trimester Scan

To know the Expected date of delivery (EDD).

To findout twin pregnancy, rule out fetal anomaly at the earliest, placental localization.

If LMP is known, add 7 to days, subtract 3 from months). Naegele’s method

A normal anteverted uterus on lower abdominal vertical scan.

The Gestational Sac:

Demonstration of a gestational sac within the uterine cavity is the earliest sign of pregnancy.
It is usually visualized from 31 days or 4 weeks of gestation using the transvaginal method, when it measures 2–3 mm in diameter. It can be identified about a week later, i.e identified at. 5 -6weeks of gestation, using the abdominal route.

Gestational Sac on Longitudinal Abdominal USG

Gestational Sac on Transverse Abdominal USG

The early gestation sac appears as a circular transonic area surrounded by a thick bright ring. It usually lies at the uterine fundus and is eccentrically placed.
The ring and the eccentric position of the gestation sac and double decidual sign are important markers for confirming an intrauterine pregnancy.

Large sac size >25mm, distorted gestational sac shape, low position suggests embryonic demise.


Diagnostic of intrauterine pregnancy
Disintegrates before 12 weeks
Seen from 7-12 weeks
More than 10 mm is abnormal
Solid echogenic yolk sac suggests fetal death

The Embryo:

The period from conception to the end of the ninth postmenstrual week

CRL used for age estimation between 5-10 weeks

Cardiac activity is seen at 5 weeks by TVS and 6 weeks by TAS.

After 9weeks to birth is fetus.

Normal fetal heart rate is 110-150 bpm

Amniotic fluid volume is 1ml at 6weeks, 31ml at 10weeks, 100ml at 13 weeks.

Crown Rump Length

Measurements are taken from the top of the head (crown) to the end of the trunk (rump) using the onscreen calipers.

Used during the 1st trimester.

Once the fetal spine can be easily identified, i.e. from about 9 weeks, this should be used as a guide in assessing true fetal length.

With increasing gestational age, the fetus is more likely to be found in a flexed position. After 12 weeks this generally makes assessment of the gestational age by CRL inaccurate.

Multiple Pregnancy:

Twins arise from the fertilization of two eggs (dizygotic twins) or a single egg (monozygotic twins). The fertilized egg is termed the zygote. All dizygotic twin pairs have separate placentas and therefore separate chorionic sacs and separate amniotic sacs.
Dizygotic are always ‘diamniotic dichorionic’ twins. The single egg of the monozygotic twins can divide into two individuals at different stages resulting in three types of monozygotic twins.

All dizygotic twins are dichorionic. However not all (approximately 60%) monozygotic twins are Monochorionic.
Dizygotic twins can be the same sex or different sex; monozygotic twins will be the same sex.

easy to see the twins in the first trimester

Ultrasound examination of multiple pregnancy in the first trimester should provide information about the number of fetuses and chorionicity of pregnancy

Apparently empty sacs in a multiple pregnancy might regress spontaneously before the embryo develops. This ‘vanishing twin’ phenomenon has been reported to affect up to 20% of multiple pregnancies.

Therefore the diagnosis of multiple pregnancies is delayed till appearance of live embryos.

Monochorionic diamniotic twins have two yolk sac, two embryo and two amnions within one chorionic sac.

Monochorionic monoamniotic twins have one amnion, one or two yolk sac and two embryos within one chorionic sac.

After 7 weeks, the presence of more than one embryo within a single gestation sac enables the conclusive diagnosis of a monozygotic monochorionic multiple pregnancy.

A case of twins at 8 weeks imaged transvaginally. Note the single chorionic cavity, which contains two amniotic cavities, confirming this is a case of monochorionic diamniotic twins

A dichorionic twin pregnancy demonstrating one sac containing a live fetus (CRL 21.4mm) and a dead twin (CRL 11.9mm) in the second sac.

Establishing Chorionicity and Amnionicity:
- Monochorionic Diamniotic twin show a characteristic T sign on ultrasound.
The presence of two separate sacs each with its own placenta is diagnostic of a dichorionic twin pregnancy.
The presence of an apparently single placenta can indicate either a dichorionic pregnancy with adjacent implantation of the two placentas or a monochorionic pregnancy with a single placenta. In such a situation chorionicity should be established.

Molar pregnancy:

Whole placenta is replaced by grossly dilated hydropic villi.
Solid collection of echoes with multiple anechoic spaces
Typical bunch of grapes appearance
No fetal part seen
Snow-storm appearance in the 2nd trimester

Ectopic pregnancy:

Clinical triad of pain, abnormal vaginal bleeding, palpable adnexal mass (amennorhoea also)
Sonography suggests live embryo in the adnexa
Tubal ectopy is usually common
Empty uterine cavity
Adnexal heterogenous mass lesion
Positive serum Bhcg
In almost all such cases, there will be fluid in the pouch of Douglas.


A second trimester ultrasound examination should be more than a means of confirming gestational age because it provides an ideal opportunity for assessing fetal anatomy and therefore structural normality .placental morphology, amniotic fluid volume is also noted.

most of the fetal anatomy can be evaluated at 18–20 weeks, it is frequently difficult to examine the fetal heart
In the majority of normal pregnancies, measurement of the biparietal diameter (BPD) and femur length (FL) provide the most accurate assessment of gestational age in the second trimester

The aims of usual second trimester scans are:

1. Determine the number of fetuses
2. longitudinal lie of the fetus
3. Measure the BPD, HC and TCD, evaluate the intracranial anatomy
4. four-chamber view of the fetal heart
5. the femur length.
6. Look for normal presence of long bones of upper and lower limbs.
7) Localization of placenta and evaluation of amniotic fluid volume.
8) Observe the fetal activity in terms of body and limb movements.

Longitudinal Axis of Fetus:

Using midline longitudinal section of the uterus.
Spine of the fetus is seen to locate the heart

Measuring the BPD, HC, FL:

- The BPD is the maximum diameter of a transverse section of the fetal skull at the level of the parietal eminences
The BPD has traditionally been the most widely used ultrasound parameter in the estimation of gestational age

- A single optimal measurement of the BPD will predict the gestational age to within ± 5 days.

It is more precise than the optimum menstrual history to ascertain the EDD.

Head circumference is also measured along with BPD using the formula for circumference of a circle.

FL is as accurate as BPD to determine gestational age, can be measure anytime after 12 weeks of pregnancy to term to confirm the gestational age determined by BPD or HC

Placental Localization:

Placenta seen as a separate organ by ultrasound by 8th week
The placental site can change relative to the internal os.
The placenta is best identified longitudinally and has more echogenic pattern compared with that of the underlying myometrium.
Placenta may be anterior walled, posterior walled, fundic, or low lying.
Placenta previa may be marginal, partial, complete.Low lying palcenta is the one which remains 0.5-5cm away from internal os. Internal os is at max 6 cm from external os.(after28wk)


Grade 0: placenta uniformly granular, echogenic.
Grade 1: chorionic plate slight indulating. Linear echogenic densities in placental substance parallel to basal plate
Grade 2: marked indentation of chorionic plate, but not upto basal plate. The basal plate contains echogenic lines in single row.
Grade 3: chorionic plate shows deep indentation upto basal layer so placenta divided into cotyledons, calcification areas are seen giving acoustic shadow. Grade3 placenta was thought to be indicator of fetal lung maturity but resent studies could not find these.
placental thickness >5cm is pathological, eg. maternal DM, Rh incompatibility, syphillis, molar pregnancy

Amniotic Fluid Volume:

From early in the second trimester most of the amniotic fluid is fetal urine. Amniotic fluid is therefore produced by the fetal kidneys and removed by fetal swallowing and subsequent absorption by the fetal bowel.
The abdomen is divided into four quarters with the ultrasound probe held in the longitudinal axis, the largest vertical pool depth in each quadrant is recorded The sum of these measurements represents the amniotic fluid index (AFI). Although the AFI is known to vary with gestational age, an AFI < 5 cm is classified as oligohydramnios and an AFI > 25 cm is classified as polyhydramnios

Fetal abnormality

IUGR may be seen with less growth.easy method is to see if the estimated date is more than 3,2 or1 week different than the date seen at USG.

Fetal death –at first trimester if chromosomal abnormality,at second trimester if nongenetic factors eg. Uterine anomalies,incompetent cervix,or placental abnormalities.signs are gas in fetal vasculature,fetal maceration,finally spalding sign.

Blighted ovum-8 to 12 wk amenorrhoea,bleeding pv,in this case sac develops but an embryo isnot formed

Can access maternal cervical competency to be good if cervix length>2cm and if int. os is <8mm,and if no fluid in endocervical canal