Multiple events occur in 1st trimester

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  Ovulation with resultant corpus luteum in ovary

  
  
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  Fertilization of ovum → zygote

  
  
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  Zygote cleavage → blastocyst → embryoblast + trophoblast

  
  
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  Implantation → intrauterine sac-like structure

  
  
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  Embryonic development, organogenesis

  
  
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  Development of placenta, umbilical cord

Embryoblast cells form embryo and amnion

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  Bilaminar embryonic disc forms when embryoblast splits into epiblast and hypoblast

  
  
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  Trilaminar disc develops by process of gastrulation, which moves cells to different locations

  
  
  
  
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    Creates 3 primary germ layers of ectoderm, mesoderm, endoderm

    
    
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    Determines axes of body

  
  

Embryonic disc lies between amnion and yolk sac

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  Disc folds on itself to form a tube surrounded by amnion

  
  
  
  
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    Yolk sac is pinched off as embryonic disc sides come together to form tube

  
  
  
  
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  Series of tubes within tubes elongate, bud, rotate, and form all major organs by end of 13th week

Trophoblast cells give rise to membranes and placenta, not embryo proper

Gestational sac initially covered in chorionic villi, atrophy of those adjacent to uterine cavity → chorion laeve

Chorionic villi adjacent to implantation site develop into mature tertiary villi

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  Tertiary villi contain fully differentiated blood vessels for gas exchange

  
  
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  This part of chorion increases in thickness and echogenicity → chorion frondosum

Maternal endometrial cells differentiate into decidual cells

Chorion frondosum + maternal decidua basalis = placenta

Embryonic disc lies between amnion and yolk sac

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  Embryo initially connected to chorion by connecting stalk

  
  
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  Umbilical cord forms from incorporation of connecting stalk, allantois, vitelline duct

Earliest visible structure is an intrauterine sac-like structure

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  Measure internal diameter (i.e., fluid component only, not echogenic wall)

  
  
  
  
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    Average of 3 orthogonal planes = mean sac diameter (MSD)

  
  
  
  
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  Sac-like structure is chorion; commonly referred to as gestational sac (GS)

  
  
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  Next visible structure is yolk sac (YS) inside GS

  
  
  
  
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    Round, distinct wall, normal size ≤ 6 mm

  
  
  
  
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  Next visible structure is embryo

  
  
  
  
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    1st visible as thickening of YS wall

    
    
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    As amnion expands double bleb sign seen with embryo inside amnion, YS attached to embryo

    
    
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    Abdominal wall closure → YS separation from embryo

    
    
    
    
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      Embryo inside amnion, YS outside amnion

    
    
    
    
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    Embryo elongates, develops clear cranial (crown) vs. pelvic (rump) polarity, limb buds appear

    
    
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    At 10 weeks gestation, embryo → fetus

    
    
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    Organogenesis complete by 13 weeks

  
  
  
  
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  Measure crown rump length (CRL) as longest axis of embryo; do not include YS

Pregnancy of unknown location (PUL)

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  Positive pregnancy test with no evidence of either intrauterine pregnancy (IUP) or ectopic pregnancy by transvaginal ultrasound (TVUS)

  
  
  
  
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    ~ 50% will resolve without definite diagnosis of IUP or ectopic

    
    
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    ~ 35% will have IUP

    
    
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    10-20% will have ectopic pregnancy

  
  

ProbableIUP

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  Intrauterine sac-like structure without YS or embryo

  
  
  
  
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    If echogenic with round or oval shape, statistically highly likely to represent IUP

    
    
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    Positive pregnancy test, smooth-walled anechoic intrauterine sac-like structure, no adnexal mass

    
    
    
    
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      99.98% probability of IUP

      
      
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      0.02% probability of ectopic pregnancy

    
    
    
    
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    If sac-like structure has pointed edges, more likely to be intracavitary blood products or abnormal GS

  
  

Definite IUP

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  Intrauterine sac-like structure with YS or embryo (regardless of cardiac activity)

Definite ectopic pregnancy

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  Extrauterine GS with YS ± embryo (regardless of cardiac activity)

Probable ectopic pregnancy

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  Inhomogeneous adnexal mass (described as blob sign)

  
  
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  Extrauterine sac-like structure (described as bagel sign)

  
  
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  Modern equipment resolution is so good that blob and bagel signs are often considered diagnostic of ectopic pregnancy

In multiple pregnancies, prognosis depends on chorionicity, which is best determined in 1st trimester

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  2 GS = dichorionic twins, 1 GS = monochorionic twins

  
  
  
  
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    Monochorionic twins can be diamniotic, monoamniotic, or conjoined

  
  

Scan entire uterus

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  Müllerian duct anomalies may result in widely separated uterine horns

  
  
  
  
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    Multiple pregnancies may occur with implantation in 1 or both horns

    
    
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    Potential to miss an IUP in 1 horn if incomplete study performed

  
  
  
  
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  Heterotopic pregnancy occurs when there is an IUP and an ectopic pregnancy

  
  
  
  
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    Scan entire pelvis to evaluate adnexal structures

    
    
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    Commonest heterotopic pregnancy is IUP + tubal ectopic

    
    
    
    
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      Incidence in assisted reproduction patients can be as high as 1:60-100

    
    
    
    
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    Pitfall : Corpus luteum (CL) may look like extrauterine GS

    
    
    
    
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      CL is intraovarian

      
      
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      With transducer pressure, CL moves with ovary, tubal ectopic will separate from ovary

      
      
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      Color Doppler does not differentiate CL from tubal ectopic; both may show ring of fire appearance of prominent peripheral flow

    
    
    
    
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    Pitfall : Some ectopics may appear intrauterine

    
    
    
    
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      Cervical, interstitial, cesarean scar pregnancies

    
    

  
  

Gestational age assessment is most accurate in 1st trimester

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  GS usually visible by 4.0-4.5 weeks from last menstrual period (LMP)

  
  
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  YS usually visible by 5.0-5.5 weeks from LMP

  
  
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  Distinct embryo with cardiac activity usually visible by 6.0-6.5 weeks from LMP

American College of Obstetric and Gynecology guidelines on redating recommend use of sonographic dates depending on menstrual age and number of days discrepancy between sonographic and menstrual dates

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  At ≤ 8 6/7 weeks, redate if > 5 days discrepancy

  
  
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  At 9 0/7 weeks to 15 6/7 weeks, redate if > 7 days discrepancy

  
  
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  At 16 0/7 weeks to 21 6/7 weeks, redate if > 10 days discrepancy

  
  
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  At 22 0/7 weeks to 27 6/7 weeks, redate if > 14 days discrepancy

  
  
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  At 28 0/7 weeks onward, redate if > 21 days discrepancy

Viable pregnancy defined as one that may potentially result in liveborn baby

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  Embryo must be visible if MSD > 25 mm by TVUS

  
  
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  Embryo of > 7 mm CRL on TVUS must have cardiac activity

  
  
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  Embryo of > 15 mm CRL on abdominal scan must have cardiac activity

Nonviable pregnancy defined as one that cannot possibly result in liveborn baby

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  Ectopic pregnancy, failed IUP

Follow-up recommendations to prove viability in probable IUP or definite IUP without embryo

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  Lack of live embryo ≥ 14days from visualization of sac without YS (probable IUP) is diagnostic of failed pregnancy

  
  
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  Lack of live embryo ≥ 11 days from visualization of sac with YS (definite IUP) is diagnostic of failed pregnancy

IUP of uncertain viability

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  Mean sac diameter < 25 mm without embryo

  
  
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  Embryo < 7 mm without cardiac activity

From 11-13 weeks, certain findings can be used to identify higher risk for aneuploidy

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  Increased nuchal translucency

  
  
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  Ductus venosus waveform

  
  
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  Nasal bone assessment

  
  
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  Facial angle

  
  
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  Tricuspid regurgitation

Cell-free DNA testing in which maternal blood is analyzed for fragments of fetal DNA to detect aneuploidy has decreased emphasis on structural findings for risk stratification

Structural malformations may be visible even in absence of markers for aneuploidy

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  Abdominal wall defect

  
  
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  Limb reduction abnormalities

  
  
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  Alobar holoprosencephaly

  
  
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  Neural tube defect

  
  
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  Congenital heart disease

Chorionic bump

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  Focal protrusion of chorion often containing low-level swirling echoes

  
  
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  Thought to be arterial bleed within chorion

  
  
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  Associated with pregnancy failure if enlarges or if multiple

  
  
  
  
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    Strong association with partial mole

  
  

Perigestational hemorrhage (PGH)

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  Hematoma in subchorionic space adjacent to GS

  
  
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  Use large field-of-view sweeps to assess size of bleed compared to sac size

  
  
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  Large PGH associated with increased risk of pregnancy loss, particularly if no living embryo

  
  
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  Early PGH (< 7 weeks) associated with higher risk of loss than later bleed

Look at uterine contour for possible müllerian duct anomaly

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  Associated with recurrent pregnancy loss, fewer successful outcomes with assisted reproduction

  
  
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  IUP may be missed if only 1 horn of bicornuate uterus is scanned

  
  
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  Increased risk of preterm birth

Document fibroid size and location

Document nabothian or Gartner duct cysts

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  Potential cause of confusion during cervical evaluation later in gestation

1st trimester is time of complex cell multiplication and differentiation with great potential for error if normal processes are not clearly understood

Negative cell-free DNA screen does not change need for attention to detail as significant birth defects may occur without chromosomal abnormality

Always use TVUS in early 1st-trimester scans for highest resolution

In stable patient with IUP of uncertain viability, wait and see; avoid premature interruption of potentially viable pregnancy

In PUL, close surveillance is mandatory with serial beta hCG measurement; repeat TVUS if levels rise

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  10-20% risk of ectopic, in which case, methotrexate administration is appropriate

  
  
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  ~ 35% will have IUP, in which case, methotrexate administration is catastrophic as it is a potent teratogen