1st Trimester
AIUM/ACOG/ACR Guidelines

Transabdominal and/or transvaginal imaging
 Appropriate labeling required
 Uterus, including the cervix and adnexa, to evaluate for
the presence of a gestational sac
 If a gestational sac seen, its location documented
 Gestational sac evaluated for presence or absence of
yolk sac or embryo
 Crown-rump length should be recorded
 Presence or absence of cardiac activity recorded
http://aium.org/resources/guidelines/obstetric.pdf
1st Trimester
AIUM/ACOG/ACR Guidelines

Fetal number reported
 In multiple gestation, amnionicity and chorionicity
documented
 Fetal anatomy appropriate for first trimester (unspecified)
 Appearance of the nuchal region
 Uterus, cervix adnexal structures, cul-de-sac evaluated



presence, location and size of adnexal masses
presence and size of leiomyomata
free fluid
http://aium.org/resources/guidelines/obstetric.pdf
1st Trimester
Establishing age of the pregnancy



For clinical (US) purposes, the first trimester is 2 weeks
0 days to 13 weeks 6 days
The terms menstrual age and gestational age are
equivalent
Embryonic age starts at fertilization (2 weeks gestational
age), concludes at the end of the 10th week (12 weeks
gestational age) and is used by embryologists to
describe human development, but is not useful in clinical
practice
1st Trimester

Pregnancy Dating

Gestational age based
on time elapsed from
start of last menses


Reported in weeks plus
days
Embryonic age based
on date of fertilization always 2 weeks less
than gestational age
1st Trimester
2 weeks 0 days to 3 weeks 0 days
Maternal Serum hCG
1st Trimester
3 weeks 0 days
4 weeks 0 days
3 weeks 2 days
1st Trimester
4 weeks 0 days
5 weeks 0 days
1st Trimester

Gestational Sac





Often visible at 3 mm (4
weeks 6 days age)
Thick echogenic rim
(chorion)
Intradecidual sign
Diameter reported as the
average of three
measurements
Enlarges approximately
1 mm mean diameter
per day
1st Trimester

Yolk Sac




Visible when GS is 10
mm (5 weeks 4 days)
First unequivocal sign of
intrauterine pregnancy
Resides in the extraembryonic space
(coelom)
Measurements not
useful
1st Trimester

Embryo



Visible at approximately
3 mm (5 weeks 6 days)
Adjacent to yolk sac
Grows at approximately
1 mm per day
1st Trimester

Embryo (Crown-Rump
Length)


Best US predictor of
gestational age between
7-12 weeks
Useful up to 14 weeks
1st Trimester

Amnion


Visible between 6 and 7
weeks
Enlarges to obliterate the
extra embryonic coelom
and “fuses” to the
chorion by 16 weeks
1st Trimester

Embryonic Heart
Activity



Visible when CRL is as
small as 3 mm (5 weeks
6 days)
Rate starts very slow,
and exceeds 160 in
normal early pregnancy
Documentation with mmode or cine preferable
to Doppler
1st Trimester
Complications
Ultrasound Predictors of Abnormal 1st Trimester Pregnancy
Criterion
“Old” Standards
“New” Standards
Gestational sac mean
diameter
If >20 mm mean
diameter and empty
If >25 mm mean
diameter and empty
Yolk sac
If absent when GS is
>10 mm
If absent when GS is
>20 mm
Heart activity
If absent when GS is
>16 mm or CRL >5 mm
If absent when CRL >7
mm
Ultrasound Obstet Gynecol 2011:38:489
1st Trimester
Complications



Threatened abortion (miscarriage) - vaginal bleeding
prior to viability
Inevitable abortion – abnormal gestational sac with no
live embryo and dilated cervix
Missed abortion (retained products of conception) –
embryo is dead for at least 8 weeks but no passage of
tissue
1st Trimester
Complications

Extra-chorionic
(subchorionic)
hematoma




Between chorion and
uterine wall (decidua)
Common in
asymptomatic patients
Some correlation of size
with clinical outcome
Size measured in 3
orthogonal dimensions
1st Trimester
Complications

Extra-chorionic
(subchorionic)
hematoma


Hematoma usually has
low level echos
Because hematoma is
extravascular, there is
no flow (unlike placental
venous sinus)
1st Trimester
Complications

Extra-chorionic
(subchorionic)
hemorrhage


But color Doppler is
misleading because
placental vascular sinuses
have flow velocity below
the threshold of most
Doppler instruments
Hematoma is differentiated
from a venous sinus with
high resolution grey scale
1st Trimester
Complications

Ectopic Pregnancy





Most common in assisted
reproduction (IVF etc.)
Presentation most common
in 1st trimester
Absence of intrauterine
gestational sac is key
Presence of an intrauterine
gestational sac does not
exclude an ectopic
Presence of blood in
peritoneal cavity
(hemoperitoneum) helpful
but not always present
1st Trimester
Complications

Tubal Ectopic


80% of ectopics in ampula
or fimbrae
Hematoma in the tube
(hematosalpinx) is subtle
and must be actively
searched for
1st Trimester
Complications

Interstitial Ectopic




3% of ectopics
Presentation commonly in
early 2nd trimester
Implantation in the tube
between the between
isthmus and endometrial
cavity
US findings are a
gestational sac adjacent to
the uterus with absent or
thin (<5 mm) myometrium
1st Trimester
Complications
1st Trimester
Multiple Gestation

Twins (“high risk”)


Perinatal mortality rate
of dizygotic (fraternal)
twins 3-7x singletons
Perinatal mortality rate
of monozygotic
(identical) twins 2-5x
times dizygotic twins



Dichorionic diamniotic
Monochorionic,
diamniotic
Monochorionic,
monoamniotic
1st Trimester
Multiple Gestation

Dichorionic



Complete chorion
around each twin
Easy diagnosis up to
12 weeks – chorion is
thick and echogenic
relative to amnion
Twin “peak” sign
1st Trimester
Multiple Gestation

Monochorionic

Diamniotic
1st Trimester
Multiple Gestation

Monochorionic

Monoamniotic
1st Trimester
Challenge