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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
Maternal Serum hCG2 weeks 0 days to 3 weeks 0 days
1st Trimester
3 weeks 0 days
3 weeks 2 days
4 weeks 0 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 extra-
embryonic 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 m-mode or cine preferable to Doppler
1st TrimesterComplications
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
Ultrasound Predictors of Abnormal 1st Trimester Pregnancy
1st TrimesterComplications
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 TrimesterComplications
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 TrimesterComplications
Extra-chorionic (subchorionic) hematoma
Hematoma usually has low level echos
Because hematoma is extravascular, there is no flow (unlike placental venous sinus)
1st TrimesterComplications
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 TrimesterComplications
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 TrimesterComplications
Tubal Ectopic 80% of ectopics in ampula
or fimbrae Hematoma in the tube
(hematosalpinx) is subtle and must be actively searched for
1st TrimesterComplications
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 TrimesterComplications
1st TrimesterMultiple 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 TrimesterMultiple 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 TrimesterMultiple Gestation
Monochorionic Diamniotic
1st TrimesterMultiple Gestation
Monochorionic Monoamniotic
1st TrimesterChallenge
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