This document provides an overview of obstetric ultrasound procedures and measurements. It discusses estimating gestational age using biometric measurements like crown-rump length, biparietal diameter, abdominal circumference, and femur length in each trimester. It also covers assessing fetal number, position, growth, amniotic fluid, and potential complications like placenta previa or abruption. Key ultrasound goals are determining gestational age, fetal well-being, and detecting any anomalies or complications.

1. Obstetric ultrasound

2. outline
• Introduction
• GA estimation in each trimester
• Estimating fetal weight
• Fetal lie & presentation
• Number of fetuses
• AFV assessment
• Biophysical measurement
• Evaluation of multiple pregnancy

3. INTRODUCTION
• The term "ultrasound" refers to sound waves of a frequency
greater than that which the human ear can appreciate, namely
frequencies greater than 20,000 cycles per second or Hertz
(Hz).
• To obtain images of the pregnant or non-pregnant pelvis,
frequencies of 2 to 10 million Hertz (2 to 10 megahertz
[MHz]) are typically required.

4. Frequency spectrum of sound
• Sound wave Frequency
• Ultrasound >20kHz
• Audible sound 20Hz to 20kHz
• Infrasound <20Hz

5. Ultrasound terms
• Anechoic
• Hypo echoic
• Hyper echoic

7. SOUND WAVES
• Sound waves are a type of mechanical vibration
• They are described in terms of their frequency, which is the
number of repetitions (ie, cycles) per second. The unit for
measuring frequency is the Hertz (Hz).
• Other characteristics of sound waves are wavelength, the
distance between excitations; the amplitude of excitation,
measured in decibels (dB);
• and the period, the time it takes for one cycle to occur,
measured in seconds.

8. • The speed with which an acoustic wave moves through a medium
is dependent upon the density and resistance of the medium.
• Media that are dense will transmit a mechanical wave with
greater speed than those that are less dense.
• As an example, the acoustic speed of a mechanical wave through
air is 331 meters/second; through water, it is 1450 meters/second;
through soft tissue, it is 1540 meters/second; and through bone, it
is 3,500 meters/second

9. Speed of sound
• Media type speed(m/s)
• Air 330
• Fat 1,450
• Water 1,450
• Soft tissue 1,540
• Bone 3,500
• Metals up to 7,000

10. BASIC PROCEDURE AND EQUIPMENT
• Patient position — In obstetrics and gynaecology, most
exams are performed with the woman in a semi-recumbent
position
• Gel — Ultrasound waves do not pass through air well;
therefore, coupling gel is necessary. The gel, which is placed
on the patient's skin for transabdominal scans and on the
covered probe for transvaginal scans,

12. Ultrasound probes

13. Limitations of US
• Don't penetrate bone
• Performs poorly when there is air b/n transducer and organ of
interest
• Depth penetration is limited /obese pts
• It is operator dependant, needs skill and experience
• Difficult to tell what is what from a fixed image

14. OBSTETRICAL SONOGRAPHY
Determine the location of pregnancy (intrauterine or extrauterine)
Estimate gestational age and date of delivery
Confirm fetal cardiac activity or fetal death
Evaluate vaginal bleeding or pelvic pain
Evaluate a significant uterine size and dates discrepancy
Diagnose suspected multiple gestation and determine chorionicity
Evaluate fetus for anomalies
Evaluate fetal weight and growth over time
As an adjunct to diagnostic or therapeutic procedures

15. Evaluate suspected gestational trophoblastic disease
Evaluate cervix for risk of preterm loss or birth
Evaluate a pelvic mass
Follow-up of a previously detected abnormality
As part of fetal aneuploidy screening
Evaluate a suspected uterine abnormality
As part of the evaluation of abnormal maternal analytes
Biophysical fetal evaluation
Evaluate suspected abnormalities of amniotic fluid volume (oligohydramnios,
polyhydramnios)
Evaluate suspected abruptio placentae

16. • A basic obstetrical ultrasound examination provides the
following information :
– Fetal number (chorionicity if multiple gestation)
– Fetal biometry
– Fetal presentation
– Documentation of fetal cardiac activity
– Placental appearance and location
– Assessment of amniotic fluid volume
– Survey of fetal anatomy

17. Prenatal assessment of gestational age
• Ultrasound is commonly used to estimate gestational age and
thereby calculate the expected date of delivery (EDD). Several
studies have demonstrated that ultrasound estimation of EDD
in the first half of pregnancy is superior to dating based on the
last menstrual period or physical examination.

18. • Indications — Sonographic estimation is particularly
important when menstrual cycles are irregular, the last
menstrual period is unknown, and in patients who conceive
while using hormonal contraception.

19. First trimester
• optimal time to obtain an estimate of gestational age is during
the first trimester when biologic variation in size from fetus to
fetus is minimal.
• First trimester sonograms can be performed via the
transvaginal (TVS) and/or the transabdominal (TAS) route. I
• In the earliest stages of pregnancy, TVS generally provides
clear and accurate images while TAS may be unable to even
detect an intrauterine gestation . Therefore, TVS is typically
used for early first trimester evaluation of the gestational sac,
yolk sac, and developing embryo.

20. Pregnancy dating in the 1st trim.
• If a patient reports no menstrual dates, US in the 1st or 2nd trim.
Should establish the estimated date of delivery.
• If US biometric measurements vary from menstrual dates by
more than 5-7 days in the 1st trim.tha US should be used to
establish the date of delivery.
• US dating of pregnancy is most accurate in the 1st trim.
• Biometric parameters:
– Gestational sac
– CRL(the most accurate)
– Yolk sac

21. • Gestational sac — Using transvaginal sonography: the
gestational sac is usually visible at 4.5 to 5 weeks of gestation,
with a double decidual sign appearing at 5.5 to 6 weeks
• The gestational sac is the first sonographic sign of an
intrauterine pregnancy
• Gestational sac confirms the presence of an intrauterine
pregnancy but not viability of the embryo.
• appears as a small, fluid filled, sac-like structure eccentrically
located within the endometrium.

22. • gestational age based upon gestational sac measurements can be
calculated in several ways.
• One standardized formula that is frequently used is the Mean Sac
Diameter (MSD), which is derived by calculating the mean of the
three orthogonal sac diameter measurements
• A simplified method of estimating gestational age is to calculate
the sum of 30 plus the sac size in millimeters; this number is
equivalent to the gestational age in days. As an example, using a
sac size of 5 mm, the calculated age would be 35 days (30+5) or
five weeks

23. A normal intrauterine pregnancy at 4
weeks’ gestation imaged using the transvaginal method.
The gestational sac measures 3 mm
• A normal intrauterine pregnancy at 4
• weeks’ gestation imaged using the
transvaginal method.
• The gestational sac measures 3 mm

24. • Mean gestational sac measurement

26. • Yolk sac — The yolk sac is the first anatomic
structure to appear within the gestational sac and
provides confirmation of an intrauterine pregnancy.
The yolk sac is spherical with a sonolucent center and
echogenic periphery .
• It can be noted initially at the beginning of the 5th
week of gestation (MSD approximately 5 mm),
although it may not appear until the MSD approaches
8 mm .
• Therefore, reasonable criteria for an abnormal
gestation are MSD of 8 mm or greater with an absent
yolk sac

27. yolk sac

28. • Crown rump length — The crown-rump length (CRL) is the
standard biometric measurement of the embryo in the first
trimester . By definition, the crown-rump length is the longest
straight-line measurement of the embryo measured from the outer
margin of the cephalic pole to the rump
• The CRL increases rapidly at a rate of approximately 1.1mm/day
• A length of at least 5 mm routinely allows visualization of the
embryo
• In general, when the CRL is under 25 mm, the GA (in days) is
CRL(mm) + 42

29. CRL

30. • Accuracy of 1st trimester US in estimating GA
plus or minus 5 days

31. Landmark
First appearance on transvaginal
ultrasound examination
Gestational sac 4.5 to 5 weeks
Yolk sac 5 weeks
Cardiac activity 5.5 to 6 weeks
Measurable crown-rump length 6 weeks
Timing of first appearance of gestational landmarks on transvaginal ultrasound

32. • Cardiac activity — If the embryo is visible, but too small to
measure adequately, detection of cardiac activity establishes a
gestational age of 5.5 to six weeks

33. 2nd and 3rd trimester
• The four standard biometric parameters commonly used to
estimate gestational age and/or fetal weight in the second
and third trimesters are:
– biparietal diameter (BPD),
– head circumference (HC),
– abdominal circumference (AC),
– and femur length (FL)
• They are typically obtained by transabdominal ultrasound
examination.

34. • Biparietal diameter — The biparietal diameter
(BPD) is the best studied biometric parameter because
it is highly reproducible and can predict gestational age
within ±7 days when measured between 14 and 20
weeks of gestation.
• Unfortunately, test performance diminishes as the
gestation progresses beyond this period . By the mid to
late third trimester, the margin of error is three to four
weeks
• The BPD is measured on a plane of section that intersects
both the third ventricle and thalami

35. Sonographic landmarks for the
measurements of BPD
• Midline falx
• Thalami
• Symmetrical appearance of both cerebral
hemispheres
• No cerebellum visualized……etc

36. BPD

37. • Head circumference — Measurement of the fetal head
circumference (HC) provides a good estimate of gestational
age on routine sonograms, but also is useful in the clinical
setting of growth disorders when other measures may not
perform well .
• The accuracy is within one week prior to 20 weeks of
gestational age
• . As with other biometric measures, test performance falls in
the second half of pregnancy and, by the late third trimester,
variability can be within three to four weeks

39. • Femur length — The femur length (FL) can be measured as
early as 10 weeks gestational age because of its size and
echogenicity. Correlation with true gestational age is within
one week prior to 20 weeks gestational age, but falls to within
2.1 to 3.5 weeks in the third trimester
• In order to optimize the measurement of the FL the whole
femur diaphysis should be seen
• Imaging of the long axis of the femur can be more technically
difficult than the BPD,AC & HC.

40. FL

41. • Abdominal circumference — The abdominal circumference
(AC) appears to have a slightly lower ability to predict
gestational age early in the second trimester than the BPD, HC,
and FL . Some of the variability may be due to error in ultrasound
technique, along with natural biologic variations.
• In the second trimester, gestational age assessment is accurate
within two weeks . In the late third trimester, variability increases
so accuracy falls to within three to four weeks .
• Due to the wide margin of error, AC is often used for estimations
of fetal weight and interval growth evaluations rather than
gestational age assessment.

42. Landmarks for AC measurement
• Circular cross section of abdomen (as circular
as possible )
• Spine seen on cross section
• Stomach bubble
• Intrahepatic portion of the umbilical vein
• Large section of fetal ribs seen
• Kidneys not be visualized

44. • The image is taken at the level of the largest diameter of the
fetal liver, denoted by the point of union of the right and left
portal veins, which has a "hockey stick" appearance

45. AC

46. Estimating fetal weight
• Estimating fetal weight is more in 3rd trim. as it
becomes important to detect fetal growth
restriction or macrosomia.
• Biometric parameters
BPD
AC
FL
HC

47. • Calculating the EFW is more accurate in the
second trimester than the third trimester.
• In the 3rd trimester, EFW is of crucial
importance to detect fetal growth restriction
or macrosomia. The estimation of macrosomia
is not very accurate and the error can exceed
10% .

48. • Several formulas are currently available, but the
one that is commonly presented in the software
of most US equipment is that developed by
Hadlock.
• AC is the most accurate & sensitive predictor of
fetal weight. It is typically the 1st biometric
marker to be affected by growth abnormalities
• BPD & HC are more precise biometric markers
of gestational age than AC & FL.

49. • Hadlock formulas
• (1) Log10 BW = 1.3598 + 0.051 (AC) + 0.1844
(FL) – 0.0037 (AC X FL), or
• (2) Log10 BW = 1.4787 + 0.001837 (BPD)2 +
0.0458 (AC) + 0.158 (FL) – 0.003343 (AC X FL)

50. FETAL LIE AND PRESENTATION IN THE UTERUS
• The lie of the fetus in the uterus is defined by
the orientation of the fetal spine to the
maternal spine.
• Determining the lie of the fetus by ultrasound
therefore requires obtaining a mid-sagittal
plane of the fetal spine.

52. • Place the transducer transversely in the lower
abdomen just above the symphysis pubis.
• The presence of a fetal head on the
ultrasound monitor confirms a cephalic
presentation and the presence of fetal
buttocks confirms a breech presentation.

55. NUMBER OF FETUSES IN THE UTERUS
• One of the most important benefits of ultrasound
in obstetrics is in its ability to identify the presence
of twins or higher order multiple pregnancy.
• The diagnosis of a twin pregnancy is first
suspected when 2 fetal heads are seen in the
uterine cavity. Then provide confirmation of twin
pregnancy by the identification of two separate
bodies and a dividing membrane when present.
Imaging both fetal heads or bodies in a single
image when feasible, is proof of a twin pregnancy.

57. Placental Localization in the Uterus
• The placenta forms at the site of the chorion
frondosum (the fetal portion of chorion) and
the decidua basalis and is first recognized
sonographically as a thickened echogenic
region by about 9-10 weeks of gestation.
• Position : in the upper uterine segment
(99.5%), either in the posterior surface (2/3)
or the anterior surface (1/3).

58. • Place the transducer in the sagittal orientation
in the right upper abdomen, just above the
uterine fundus and scan longitudinally
towards the lower right abdomen.
• Repeat the same steps in the mid and left
abdomen.

62. PLACENTA PREVIA
• The term placenta previa describes a placenta
that covers part or all of the internal cervical os.
• Has 4 types
For pregnancies at less than 16 weeks of
gestation, diagnosis of placenta previa is
overestimated.
• The transvaginal ultrasound should be used as
the primary mode of imaging for the diagnosis
of placenta previa.

63. low-lying posterior placenta

64. placenta previa

65. MORBIDLY ADHERENT PLACENTA
• The term morbidly adherent placenta implies
an abnormal implantation of the placenta into
the uterine wall and this term has been used
to describe placenta accreta, increta, and
percreta.
• About 75% of morbidly adherent placentas
are placenta accretas, 18% are placenta
incretas, and 7% are placenta percretas.

66. SONOGRAPHIC FINDINGS OF PLACENTA ACCRETA
• Gestational sac implanted in the lower uterine
segment
• Multiple vascular lacunaes in the second trimester
• Multiple vascular lacunaes in the third trimester
(“Swiss-cheese appearance”)
 Retroplacental myometrial thickness of less than 1
millimeter
 Extension of villi into myometrium, serosa or
bladder

67. presence of multiple placental lacunae
(arrows).

68. PLACENTAL ABRUPTION
• . Unlike placenta previa where the sensitivity for
diagnosis by ultrasound is almost 100%, in
placental abruption the sensitivity of ultrasound
in visualizing hemorrhage is reported to be
approximately 50%.
• Ultrasound findings in placental abruption will
show a slightly hypoechoic mass either
retroplacental or behind the membranes at the
edge of the placenta that mimic an organized
blood clot.

69. presence of a blood clot (asterisk and
arrows)

70. Assessment of amniotic fluid volume
• The primary source of amniotic fluid in the 2nd & 3rd trme.of
pregnancy is fetal urine.
• Amniotic fluid volume (AFV) should be assessed either
qualitatively or quantitatively at every antenatal ultrasound
examination.
• Ultrasound examination is the only practical clinical method of
assessing amniotic fluid volume.
• There are two techniques commonly used for estimation of
amniotic fluid.

71. • 1.Single deepest pocket technique — The single deepest
pocket (SDP) measurement refers to the vertical dimension of
the largest pocket of amniotic fluid not containing umbilical
cord or fetal extremities and measured at a right angle to the
uterine contour.
• The SDP measurement has been interpreted as follows :
– Oligohydramnios — depth of 0 to 2 cm
– Normal — depth of 2.1 to 8 cm
– Polyhydramnios — depth greater than 8 cm

73. • 2.Amniotic fluid index — The amniotic fluid index (AFI)
measurement is calculated by first dividing the uterus into
four quadrants using the linea nigra for the right and left
divisions and the umbilicus for the upper and lower
quadrants. The maximum vertical amniotic fluid pocket
diameter in each quadrant not containing cord or fetal
extremities is measured in centimeters; the sum of these
measurements is the AFI.
• The AFI can be interpreted according to the following
thresholds :
– Oligohydramnios — 0 to <5 cm
– Normal — 5 to 25 cm
– Polyhydramnios — greater than 25 cm

75. AFI

76. Biophysical profile
• the fetal biophysical profile score (BPS or BPP) refers to the
sonographic assessment of four discrete biophysical variables:
– Fetal movement
– Fetal tone
– Fetal breathing
– Amniotic fluid volume
– Results of nonstress testing
• Each of these five parameters is given a score of 0 or 2 points,
depending upon whether specific criteria are met

77. Criteria for the biophysical profile
Nonstress test: 2 points if reactive, defined as at least 2 episodes of FHR accelerations
of at least 15 bpm and at least 15 seconds duration from onset to return associated
with fetal movement within a 30 minute observation period
Fetal breathing movements: 2 points if one or more episodes of rhythmic breathing
movements of ≥30 seconds within a 30 minute observation period
Fetal tone: 2 points if one or more episodes of extension of a fetal extremity or fetal
spine with return to flexion
Amniotic fluid volume: 2 points if a single pocket of fluid is present measuring at least
2 cm . However, some clinicians use other criteria such as the amniotic fluid index.
Fetal movement: 2 points if three or more discrete body or limb movements within 30
minutes of observation

78. Parameter Score of 2 Score of 0
NST Reactive Non-reactive
FBM One episode,>30sec in
30 min
Abscent , <30sec
GBM 3 movt/30min < 3 movt/30min
FT 1 episode of extension
back to flexion
No , partial flexion
AFV SVP measuring2cm or
more
No AFV, or less than 2cm
Scoring of BPP

79. • The presence of these biophysical variables implies absence of
significant central nervous system hypoxemia/acidemia at the
time of testing.
• By comparison, a compromised fetus typically exhibits loss of
accelerations of the fetal heart rate (FHR), decreased body and
breathing movements, hypotonia, and, less acutely, decreased
amniotic fluid volume.

80. • The clinical value of the cumulative fetal BPS is that it is non-
invasive, easily learned and performed, and an accurate means
for predicting the presence of significant fetal acidemia, which
is the most common cause of fetal death or damage
• . Seventy to 90 percent of late fetal deaths display evidence of
chronic or acute and chronic compromise prior to demise .
Sonographic detection of signs of fetal compromise can allow
appropriate intervention that ideally will prevent adverse fetal
sequelae.

81. • INTERPRETATION, MANAGEMENT, OUTCOME —
BPS results are interpreted as follows:
– 10/10 or 8/10 (includes 2 points for amniotic fluid): risk of
developing fetal asphyxia within one week if no intervention is
low (about 1/1000).
– 6 or 8/10 (0 points for amniotic fluid): when amniotic fluid
volume is decreased, the risk of developing fetal asphyxia within
one week if no intervention is increased at 89/1000.
– 6/10 (includes 2 points for amniotic fluid): equivocal test,
significant possibility of developing fetal asphyxia cannot be
excluded. Repeat test within 24 hours to see if one of the absent
acute variables returns to normal or deliver if at or near term.
– 0 to 4/10: risk of fetal asphyxia within one week if no
intervention is 91 to 600/1000. Delivery is usually indicated.

82. • Indications for BPP:
– Diabetes
– Hypertensive disorders
– Fetal growth restriction
– Amniotic fluid abnormalities
– Post term pregnancy
– Multiple pregnancy
– Prior stillbirth
– Obstetric haemorrhage

83. • Modified BPP- use of AFI and NS
Limitation of BPP
• Time consuming
• Require U/S and good sonographer
Cont..

84. Multiple pregnancy
• US is an integral part of the diagnosis & management
of twin pregnancies.
• Benefits of US
 Diagnosis of twin
 Determine chronicity of placenta
 Evaluation of fetal anatomy
 Detection of fetal growth abnormality
 Fetal surveillance
 Assessment for the presence of complication
 Determining fetal presentation

85. Zygosity
• Twins that arise from the ovulation and fertilization of two
eggs (dizygotic twins) or a single egg (monozygotic twins)
• All dizygotic twin pairs have separate placentas and therefore
separate chorionic sacs and separate amniotic sacs. They are
almost always ‘diamniotic dichorionic’ twins.

86. • The single egg of the monozygotic twins can
divide into two individuals at different stages
resulting in three types of monozygotic twins:
 Dichorionic diamniotic (DCDA)
 Monochorionic diamniotic (MCDA)
 Monochorionic monoamniotic (MCMA) –

87. DETERMINING TWIN PLACENTATION BY ULTRASOUND
• Ultrasound in the first trimester of pregnancy is very accurate in
determining chorionicity in twin pregnancies.
• The characteristic of the dividing membrane between the 2
gestational sacs, when present, is the most accurate way for
determining chorionicity in twin gestation.
• If the placenta appears to fill the junction between the
membranes at its insertion into the placenta, resulting in a thick
wedge-shaped configuration (Lambda, Delta or twin-peak sign),
this is diagnostic of dichorionic / diamniotic placentation .
• In monochorionic pregnancies, the membranes attach to the
uterine wall in a thin T-shaped configuration without any
placental tissue at its insertion site.

90. • Two separate placentas suggest dizygosity.
• In pregnancies with a single placental mass, it may be difficult
to identify chorionicity. Identification of a thick dividing
membrane—generally 2 mm or greater—supports a presumed
diagnosis of dichorionicity.
In contrast, mono-chorionic pregnancies have a dividing
membrane that is so thin it may not be seen until the second
trimester. The membrane is generally less than 2-mm thick,
reveals only two layers.

91. Chorionicity and amnionicity
• The assessment of chorionicity and amnionicity is important
because mono-chorionic twin pregnancies are at high risk of
developing various pregnancy complications
• Monochorionic di-amniotic twin pregnancies carry a 25% risk of
twin–twin transfusion syndrome (TTTS) and therefore require
more careful antenatal surveillance than dichorionic twin
pregnancies, which do not have this risk.
• Monochorionic mono-amniotic twins are not at increased risk of
TTTS but have a high risk of cord accidents due to cord
entanglement

92. Monochorionic diaminotic twins

93. • In 2nd /3rd trimester US is less accurate
Determine fetal gender
Assess for location & number of placenta
masses
Assess the thickness of dividing membranes
Counting the layers of dividing membrane
Twin peak sign……if seen 100% accurate
Another important use of US in multiple
gestation is to identify fetal presentations.

94. Thank you