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First Trimester Ultrasound

Last updated 08.03.13

 

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Index

 

Definition

-  Ultrasound scan performed between Day 1 & 13/40 of pregnancy

 

Indications

-  Confirm presence of intra-uterine pregnancy

-  Dating scan

-  Suspected ectopic pregnancy

-  Vaginal bleeding

-  Pelvic pain or mass

-  Uterine abnormalities

-  Evaluate for multiple gestations (twins..)

-  Confirm foetal cardiac activity

-  Assess for high risk foetal anomalies

-  Nuchal translucency (NT)

-  Routine pregnancy scan

-  Suspected Hydatiform mole

 

Features

- Transabdominal (TA) or transvaginal (TV)

-  If TA inadequate should perform TV

- Routinely all pregnant women offered NT scan

 

Procedure

-  Patient to have full bladder if possible

-  Estimate gestation age based on LMP or time of conception (IVF..)

-  Document Beta HCG

-  Uterus, cervix & adnexa examined for gestational sacs

-  Gestational sac should be visible from 4.5 to 5 weeks gestation using high freq US

-  Beta HCG >1,500 (2,000) should see gestational sac

-  If sac seen without fetus must DDx pseudo sac

-  Echogenic trophoblast rim & yolk sac support gestational sac

-  If patient pregnant & gestational sac not seen thoroughly examine adnexa for ectopic

-  Gestational age

-  Based on CRL +/- BPD

-  Used for dating if no earlier US

-  Fetal cardiac activity

-  Often visible from 5-6/40

-  Should be seen once CRL >7mm else miscarriage

-  If fetal heart absent should observe area for 30sec+ to confirm absence

-  Number of gestations

-  First trimester optimum time to determine chorionicity of multiples (twins..)

-  Should note presence of separate sacs, thickness of intervening membranes & shape of junction with placenta

-  Number of placentas can be determined in late first trimester

-  Anatomy appropriate for gestation

-  9/40: head, trunk, limbs

-  10/40: ossification of some long bones, jaw & skull

-  11/40: stomach, spine, ossified cranium, four chamber heart

-  12/40: kidneys, bladder, gut herniation no longer present

-  Maternal anatomy

-  Examine each ovary, corpus luteum

-  Uterine position & shape + pathology

-  Nuchal translucency (NT)

 

Results

-  Uterus, cervix & adnexa examined for gestational sacs

-  DDx pseudogestational sac (ass w ectopics)

-  +/- presence of yolk sac or embryo

-  Mean sac diameter can be used to date gestation

-  +/- CRL

-  More accurate estimator of gestational age than sac diameter

-  Presence or absence of foetal cardiac activity

-  TV scan usually detect cardiac motion when embryo >5mm

-  If embryo <5mm may need repeat scan to confirm cardiac activity

-  Number of gestations: twins..

-  Amnionicity, chorionicity

-  Anatomy appropriate for gestation

-  Assessment of uterus, cervix, adnexa & pouch of Douglas

-  Presence, location & size of

-  Adnexal masses

-  Leiomyomata

-  Pouch of Douglas

-  Presence or absence of free fuid

-  Blood: low level echoes (liquid), hetrogenous (solid)

-  Ascities, exudate: anechoic (black on US)

-  Nuchal translucency (NT)

-  CRL must measure btwn 45-84mm (11-14/40)

-  Algorithim for Downs syndrome risk

 

 

 

References

 

AIUM Practice Guidelines: Obstetric US, October 1 2007

ASUM: Guidelines for the Performance of Frist Trimester Ultrasound, July 2005