This document discusses ultrasound in the first trimester of pregnancy. It defines the first trimester as the first 13 weeks following the last menstrual period. The goals of first trimester ultrasound are listed as assessing for intrauterine/extrauterine pregnancy location, detecting embryonic cardiac activity, identifying blighted ovum, missed abortion, multiple gestation, estimating gestational age, and screening for congenital abnormalities. The document then discusses ultrasound findings expected at different gestational ages from the conceptus period through the embryonic period. Key findings discussed include the intradecidual sign, double decidual sac sign, gestational sac, yolk sac, cardiac activity, and estimation of gestational age. Complications discussed include ectopic pregnancy,

1. USG IN 1ST TRIMESTER
Dr Ranjit Singh Lahel
Resident (Radiodiagnosis)

2. DEFINING FIRST TRIMESTER
• The first trimester of pregnancy is defined as
the:-
13 weeks following the first day of the last
menstrual period (LMP)
• Embryologically
• Ovarian period -the first 1 to 2 weeks
• Conceptus period (3–5 weeks menstrual age),
• The embryonic period (6–10 weeks), and
• The fetal period (11–12 weeks)

3. GOALS
• To look for intra / extra uterine gestation.
• Embryonal cardiac activity.
• Blighted ovum.
• Missed abortion.
• Multiple gestation and their status.
• Estimation of gestational age.
• Congenital abnormalities
• Pregnancy with other masses

4. CONCEPTUS PERIOD

5. CONCEPTUS PERIOD
• Human development begins with fertilization
and formation of the zygote.
• As the zygote passes along the fallopian tube
toward the uterus, it undergoes cleavage into
several smaller cells termed blastomeres.
• Approximately 3 days after fertilization a ball
of 12 or more blastomeres forms the morula
and enters the uterus.

6. • A cavity soon forms within the morula converting
it into a blastocyst .
• The blastocyst is composed of three components:
• the embryoblast -which gives rise to the embryo
and some extraembryonic structures
• the blastocystic cavity, and
• the trophoblast -a thin outer layer of cells that
forms extraembryonic structures and the
embryonic part of the placenta.

7. • By the end of the third menstrual week, the
blastocyst is superficially implanted in the
endometrium ( between day 20-23)
• During the fourth menstrual week, the
blastocyst completes its implantation into the
endometrium,which is now referred to as the
decidua.

8. OPTIMAL CONDITION FOR
IMPLANTATION
• Optimal conditions of implantation
endometrial thickness greater than 7 mm

9. Implantation of the blastocyst into endometrium. Entire conceptus is approximately 0.1 mm at
this stage. Almost completely implanted blastocyst at about 23 days

10. INTRADECIDUAL SIGN ( 4.5 – 5 weeks TVS)
• In 1988 Yeh described the intradecidual sign.
• This term refers to an echogenic area
representing the implanted blastocyst that is
embedded in the thickened decidua and thus
is eccentrically located on one side of the
uterine cavity

11. INTRADECIDUAL SIGN-
The chorionic fluid in the sac is just barely visible

12. The endometrium in the pregnant state is actually called
the decidua capsularis, decidua vera, and decidua basalis

13. DOUBLE DECIDUAL SAC SIGN
• This was described as two concentric echogenic
rings of tissue surrounding the intrauterine sac
that protrudes into the uterine cavity.
• This morphologic appearance virtually excludes
ectopic pregnancy.
• The two concentric rings represent the apposing
surfaces of the decidua capsularis that surrounds
the developing gestational sac and the decidua
vera that lines the uterine wall on the opposite
side of the implantation site.

14. Double decidual sign

15. GESTATIONAL SAC
• First visualized at :
4.5-5 weeks –TVS
5 week – TAS
• The gestational sac is an anechoic space
surrounded by a hyperechoic rim of trophoblastic
tissue.
• Before other structures are visualized in the
gestational sac, it is the only means of estimating
gestational age and can therefore be measured.

16. MSD

17. •generally irregularly-shaped,pointed edges and/or filled with debris
•does not demonstrate a yolk sac
•centrally located, rather than eccentrically located
PSEUDOGESTATIONAL SAC is the concept that a small
amount of intrauterine fluid in the setting of a positive
pregnancy test and abdominal pain could be erroneously
interpreted as a true gestational sac in ectopic pregnancy.

18. YOLK SAC
is the first anatomical structure identified within the
gestational sac
Role:
1.Providing nutrients,
2.Serving as the site of initial haematopoiesis,
As the pregnancy advances, the yolk sac progressively
increases from the 5th to end of the 10th gestational week,
following which the yolk sac gradually disappears and is often
sonographically undetectable after 14-20 weeks.

19. The primary yolk sac forms at 23 days and gradually disappears as the
secondary yolk sac develops at 27 to 28 menstrual days ( visible on USG)

20. Formation of secondary yolk sac with extrusion of primary yolk sac.

21. • The first confirmatory sign of an intrauterine
gestation is the visualization of the secondary yolk
sac, which is often seen attached to a stalk
• It appears as a circular thick walled echogenic
structure with an anechoic centre within the
gestational sac, but outside the amniotic membrane
• The Yolk sac should always be visualized at
MSD of 20 mm by TAS
MSD of 8 mm by TVS

22. Yolk sac

23. • Upper normal yolk sac diameter between 5 and
10 weeks menstrual age is 5.6 mm.
• It should not be calcified or misshapen.
• Yolk sacs larger than 6 mm are usually indicative
of an abnormal pregnancy
• Yolk sac ≤ 2 mm at 8 – 12 wks: Poor outcome
• In addition, the number of yolk sacs can be
helpful in determining amnionicity of the
pregnancy.

24. Yolk sac = 9.8 mm, 9w0d
Large yolk sac

25. Two yolk sacs

26. DOUBLE BLEB SIGN
• Earliest evidence of an embryo is the double-bleb sign.
• There is visualisation of a gestational sac containing a yolk
sac and amniotic sac giving an appearence of two small
bubbles . The embryonic disc is located between the two
bubbles.
• It is an important feature of an intrauterine pregnancy
and thus distinguishes a pregnancy from a
pseudogestational sac

27. DOUBLE BLEB SIGN

28. The classic presentation is with abdominal pain and bleeding.
ECTOPIC PREGNANCY refers to the implantation of a fertilised ovum
outside of the uterine cavity

29. Features
1. Live embryo in adnexa.
2. Empty uterus
3. Pseudo sac
4. The presence of free intraperitoneal fluid / POD in
the context of a positive beta HCG and empty uterus is
~70% specific for an ectopic pregnancy
~63% sensitive for ectopic pregnancy
5. Simple adnexal cyst: 10% chance of an ectopic
6. Complex extra-adnexal cyst/mass: 95% chance of a
tubal ectopic (if no IUP)

30. Ring of fire sign: can be seen on colour Doppler in a tubal
ectopic, but can also be seen in a corpus luteum

31. EMBROLOGICAL PERIOD
• Embryo should always be visualized at
MSD of 25 mm by TAS
MSD of 16 mm by TVS
• Major changes occur in body form beginning
at 6 menstrual weeks.
• Essentially all internal and external structures
present in the adult form start forming during
the embryonic period.

32. CVS
• By the end of the eighth week the heart
attains its definite form.
• The peripheral vascular system develops a
little later and is completed by the end of the
tenth week

33. CARDIAC ACTIVITY
• Cardiac contractions starts at 36 to 37 days MA
• Should always be seen by TAS once embryo is
visualized (8 weeks MA/MSD= 25mm)
• Should always be seen by TVS once CRL is 5 mm.
• On endovaginal sonography,absent cardiac activity
in an embryo having a CRL of greater than 5mm
indicates embryonic demise.

34. FHR SHOULD BE RECORDED FOR CRL >5mm
Before 6 wks : 100 to 115 BPM
8 wks : 144 to 160 BPM
After 9 wks : 137 to 144 BPM
Embryonic bradyarrythmia : Doubilet and
Benson
CRL HR (b/min) Demise
5 mm <80 100%
80-90 64%
90-99 32%
100 11%
< 10mm <100 100%
< 15 mm <110 100%

35. GI SYSTEM
• The primitive gut forms during the sixth week.
• The midgut herniates into the umbilical
cord beginning from the eighth week until the
end of the twelfth week .
The temporary herniation slowly resolves as the
gut returns to the expanding abdominal cavity.
This normal phenomenon is not seen beyond
the first trimester.

36. 2 sacs are clearly visible.
The outer chorion with the developing placenta and the inner amnion which will
"inflate" with the production of fetal urine,to adhere to the chorion obliterating the
residual yolk sac.
The normal small mid-gut hernia into the cord is visible. This is the result of normal
midgut proliferation and will resolve by 11 weeks as the fetus lengthens. This
physiological occurrence should not be confused with an omphalocele.

37. CNS
• The open rhombencephalon is an important finding.
• The rhombencephalon of the developing brain is visible
as a prominent fluid space posteriorly. This should not
be mistaken for neck oedema or other pathology.
• This is seen at 8 to 10 menstrual weeks & eventually
develops into the normally proportioned fourth
ventricle after the eleventh menstrual week

38. Rhombencephalon

39. ESTIMATION OF AGE IN FIRST
TRIMESTER
• Gestation sac-
• From 5 weeks onwards
• Importance-first structure
• Accuracy-1wk

40. • CRL is the method of choice during first trimester.
• ACCURACY:- 5-7 days
• Caution –fetus should be in flexed position.

41. By the end of the first trimester,
measurement of the biparietal diameter
(BPD) becomes more accurate than
the CRL,
which by that time reflects errors
associated with fetal flexion and
extension

42. EARLY PREGNANCY FAILURE
• Embryonic cardiac activity
• Single most imp. factor

43. GESTATIONAL SAC FEATURES
• Abnormal gestational sac:
- BY TVS - GS ≥ 16mm & no Embryo
OR GS ≥ 8mm & no Yolk Sac
- BY TAS - GS ≥ 25mm & no Embryo
OR GS ≥ 20mm & no Yolk Sac
• Distorted sac shape
• Low position in endometrial cavity
• Thin trophoblastic reaction (<2mm)

44. Calcified yolk sac

45. ON TVS , GESTATIONAL SAC >16 MM WITHOUT EMBRYO IS ABNORMAL
Blighted Ovum : Anembryonic pregnancy
MSD: 18mm

46. HEART RATE PREDICTORS OF ABNORMAL
OUTCOME
• SLOW HEART RATE MAY PREDICT IMPENDING
DEMISE.
• FHR <80 AT CRL <5mm
• FHR <100 AT CRL<10mm
• FHR <110 AT CRL<15mm

47. SUBCHORIONIC HAEMORRHAGE (SCH) blood collects between
the uterine wall and the chorionic membrane in pregnancy. It is a
frequent cause of first and second trimester bleeding.
A subchorionic haemorrhage places the gestation at increased risk of:
placental abruption , preterm labour

48. NUCHAL TRANSLUCENCY is a finding during a specific period in
the late first trimester (11.3-13.6 weeks)
It is a collection of fluid under the skin at the back of fetus
neck
Pathology
Increased nuchal translucency is related to dilated lymphatic
channels and is considered a nonspecific sign of more
generalised fetal abnormality.
NUCHAL TRANSLUCENCY

49. Associations
Thickening of the nuchal translucency can be associated with
a number of anomalies, including:
1. Aneuploidy
Trisomies (including Down syndrome)
Turner syndrome
2. Non-aneuploidy structural defects and syndromes
Congenital diaphragmatic herniation
Congenital heart disease
Omphalocoele
Skeletal dysplasias
VACTERL association (vertebral defects, anal atresia, cardiac defects,
tracheo-esophageal fistula, renal anomalies, and limb abnormalities)

50. Nuchal lucency is measured on a sagittal image through the
fetal neck.
Technique:
(a) The fetus must be in mid sagittal imaging plane (the
vertebral column should be facing the bottom of the screen),
(b) Following structures must be seen to confirm correct mid
sagittal position:
two tiny parallel echogenic lines
tip of the nose
nasal bone
hard palate
diencephalon

51. Magnification such that only fetal head and upper thorax
included in the image: enabling 1 mm changes in
measurement possible
1. Fetal head should not be extended or flexed
2. Fetus should be floating free of the uterine wall
i.e. amniotic fluid should be seen between its back and the
uterus; this is to not mistakenly measure the distance to the
amniotic membrane or uterine wall
3. Only the lucency is measured (differing from nuchal
thickness) .The calipers are put inside the hyperechoic edges
4. The widest part of the translucency should be measured

52. Assessment
Values obtained when CRL is between 45-84 mm (11.3- 13.6
weeks) may be used for combined first trimester screening
Interpretation
The rate of aneuploidy when the nuchal translucency is <2
mm is less than 1%.

53. Correlation with serum markers
To increase the clinical accuracy of nuchal lucency, it can be
correlated with serum markers such as:
Maternal B-HCG
Alpha-fetoprotein (AFP)
Pregnancy-associated plasma protein A (PAPP-A)
Oestriol/Estriol
The combination of nuchal translucency thickness, PAPP-A,
and hCG detects 87% of cases of trisomy 21 at 11 weeks, 85%
at 12 weeks, and 82% at 13 weeks, with a 5% false positive
rate

55. (GTD) results from the abnormal proliferation of trophoblastic tissue
and encompasses a wide spectrum of diseases,
A common characteristic of all gestational trophoblastic disease is an
abnormal proliferation of trophoblast, but different components
predominate in different tumours.
1. Hydatidiform mole
complete mole ( absence of fetus)
partial mole ( abnormal fetus)
2. Invasive mole
3. Choriocarcinoma (gestational choriocarcinoma )
4. Placental site trophoblastic tumour (PSTT)
5. Epithelioid trophoblastic tumour (ETT)
GESTATIONAL TROPHOBLASTIC DISEASE

56. Due to their aggressive growth characteristics, invasive moles are
considered locally invasive non-metastasising neoplasms.
May be seen as an echogenic vascular mass invading the
myometrium. Colour Doppler interrogation will show high
velocity

57. Sonographic appearances:
-from a seemingly empty uterus to a large, echogenic mass of tissue
filling the endometrial canal.
-The presence of focal increased vascularity is of great importance in
distinguishing between blood clots and RPOC
RETAINED PRODUCTS OF CONCEPTION

58. To determine growth of twin pregnancy : the twin on left did not grow normally
( 9 weeks POG )
MONITOR GROWTH OF TWIN PREGNANCY

59. (CVS) is an antenatal procedure for prenatal diagnosis of chromosomal
or genetic disorders in the fetus.
It entails getting a sample of the chorionic villus (placental tissue) and
testing it.
Potential risks
-Miscarriage: 0.5-4% 1
-Amniotic fluid leakage
-Possible limb reduction defects: not proven in randomised controlled
trials
CHORIONIC VILLUS SAMPLING (CVS)

60. A transabdominal or transcervical approach is selected
depending on the position of the placenta to avoid entering
the amniotic cavity.
It is performed under ultrasound guidance and takes during
the 10-12 weeks of gestation,

61. STRUCTURAL ANOMALIES

62. US APPROACH AND SCANNING
PLANES
• The screening examination for anomalies of
the cns is performed by abdominal
ultrasound.
• Coronal and sagittal views are more difficult to
obtain, and often require a transvaginal scan.

63. VETRICULOMEGALY
• Incidence. High: 0.3–1.5 per 1000 births.
• Ultrasound diagnosis:-
• Axial transventricular view
• Uni- or biventricular dilatation ≥ 10 mm.
• It can be isolated or associated with other
congenital (DWM, corpus callosum agenesis)
or acquired (hemorrhage, infections) CNS
anomalies.

64. VENTRICULOMEGALY

65. HOLOPROSENCEPHALON
• The term ‘holoprosencephaly’ refers to a
group of complex abnormalities of the
forebrain deriving from a failed cleavage of
the prosencephalon that yields an incomplete
division of the cerebral hemispheres.

66. ALOBAR ( most severe)
SEMILOBAR
LOBAR

67. DANDY–WALKER MALFORMATION
• The term Dandy–Walker malformation describes a
malformation consisting of a cystic enlargement of
the fourth ventricle associated with partial or
complete agenesis of the vermis.
• The characteristic signs
• (i) complete or partial agenesis of the vermis
• (ii) cystic dilatation of the fourth ventricle
• (iii)enlarged posterior fossa with upward
displacement of the tentorium; and

68. DWM

69. ACRANIA
It is characterized by absence of the cranial vault and
the cerebral hemispheres.
• Includes two subtypes
• Exencephaly and Anencephaly (The abnormality is
absence of the bony calvarium)
• The former shows relative normal amounts of
abnormally developed cerebral tissue whereas the
latter is characterized by total absence

70. In a first-trimester fetus with exencephaly/anencephaly,
in the coronal plane the cerebral lobes will appear as
two semicircular structures above the orbits, floating in
amniotic fluid.
This finding has been referred to as the “Mickey Mouse”
sign and can be used for accurate diagnosis
of anencephaly late in the first trimester

71. Coronal scan of anencephalic fetus at 11 weeks’
gestational age shows a large, irregular cranial end
inferiorly with no visible echogenic calvarium.

72. CEPHALOCELE
• Cephalocele is characterized by protrusion of
intracranial structures through a cranial bone
defect.
• The herniated anatomic structures can consist
of meninges only (meningocele) or meninges
plus cerebral tissue (encephalomeningocele).
• The most common location is occipital

73. CEPHALOCELE

74. -Cystic hygromas are fluid-filled sacs caused by blockages in the
lymphatic s.
-They usually form between the ninth and 16th week of pregnancy.
-Approximately half of all fetuses with a cystic hygroma have
chromosomal abnormalities.
CYSTIC HYGROMA

75. OMPHALOCELE
• Definition:- Omphalocele is a defect in the
closure of the abdominal wall that also
involves the cord insertion.
• The herniated organs are wrapped in a two-
layered sac, with the two layers being the
peritoneum and the amnion.
• The incidence at birth is 1/4000 live births.

76. USG FINDINGS
• An omphalocele is sonographically
represented by a bulging structure that
• (i) arises from the anterior abdominal wall
• (ii) contains some abdominal viscera (liver
and/or bowel); and
• (iii) presents the cord insertion on its
convexity

77. D/Ds
• The differential diagnosis should include other
abdominal wall defects and the physiologic
herniation in the cord that disappears
completely after the 11th completed week of
gestation.
• In an omphalocele, there is a sac containing
the viscera, whereas in gastroschisis, the
viscera float freely in the amniotic fluid.

78. GASTROSCHISIS
• Gastroschisis is characterized by a
paraumbilical defect of the abdominal wall
through which bowel loops herniate to float
freely in the amniotic fluid.

79. USG FINDINGS
• Recognition of freely floating bowel outside the
fetal abdomen .
• Inspection may lead to identification of the right
para-umbilical wall defect through which the
bowel herniates.
• Usually, the defect is small (< 2 cm), and this is
responsible for the occurrence of bowel
infarction due to torsion and/or compression of
the mesenteric pedicle on the rim of the defect.

80. GASTROSCHISIS

81. Adnexal cystic masses less than 5 cm in
diameter in the first trimester are usually
follicular or corpus luteum cysts and almost
always resolve spontaneously.
ADNEXAL MASSES

82. Hemorrhagic corpus luteum cyst at 6 weeks.
The filamentous bands within the cyst are consistent with hemorrhage. There is
also a paraovarian cyst , which is echolucent.
The vascularity is a typical ring of fire with flow in the wall around the cyst.
Corpus luteum cysts usually regress or have decreased in size on follow-up
sonographic examination at 16 to 18 weeks. Cystic masses that persist should be
followed.