PREGNANCY DATING

PREGNANCY TESTS

  • High sensitivity urine pregnancy test (HSPT):
    • Is a simple, accurate, inexpensive urine test available over the counter in pharmacies or clinics.
    • Qualitative: most detect hCG at urine concentrations of 20-25 mIU/mL.
    • Usually positive by cycle day 32-35 (95% of pregnancies).
    • May remain positive for up to 4-6 weeks following a complete, uncomplicated abortion (therefore, generally not useful in monitoring completion).
  • Serum quantitative hCG test:
    • Detects serum levels of hCG as low as 2-10 mIU/mL.
    • Serial quantitative measurements are often used to evaluate for ectopic gestations, early pregnancy loss, or to follow molar pregnancy. May be used as an adjunct to monitor completion of abortion when there is clinical concern.
    • Wide variability in hCGs exist for any gestational age; therefore, not useful in determining EGA. Initial rapid decline in levels post-abortion (by 50% in 48 hours, or 80% in 7 days), followed by a slower decline for several weeks.
    • See Chapter 8 for details on the clinical utilization of serum hCG.
  • Other hCG assays in limited use:
    • Low sensitivity urine test (detects hCG of at least 1000-2000 mIU/mL)
    • Multi-level pregnancy test (MLPT; a graduated urine test being researched for at-home medication abortion follow-up).

BIMANUAL EXAM

Dating by uterine size in centimeters
  • After 4 weeks, uterus increases by approximately 1 cm per week
  • After 12 weeks, uterus rises out of pelvis
  • At 15-16 weeks, uterus reaches midpoint between symphysis and umbilicus
  • At 20 weeks, uterus reaches umbilicus
  • After 20 weeks, fundal height from pubic symphysis in centimeters approximately equals weeks
Dating by uterine size in fruit comparisons lemon
medium orange
grapefruit
5-6 weeks
7-8 weeks
9-10 weeks
Limitations to bimanual sizing:
  • Abdominal scarring (multiple cesareans); less uterine mobility*
  • Fibroids*
  • Multiple gestations*
  • Uterine retroversion
  • Obesity

*Consider US guidance or additional management

ULTRASOUND: OVERVIEW, METHODS, TIPS & IMAGES

Ultrasound can be used to estimate gestational age, determine the location of the pregnancy, and / or provide procedural support. US is not a requirement for uterine aspiration, however, learning its benefits and limitations will help strengthen clinical acumen.

Clinicians who use US should understand the discriminatory level (DL). This is the level of serum hCG at which a singleton viable IUP should be visible on US, although there is a lack of consensus regarding the actual hCG level (Connolly 2013), and some question using one hCG level to guide management in a desired pregnancy.

Transvaginal US must be performed if an IUP is not identified on transabdominal US.

Transvaginal Probe Transabdominal Probe
  • More invasive
  • Better view with empty bladder
  • Easier to detect earlier pregnancy
  • Better resolution but less depth
  • Probe usually 7.5 -10 mHz
  • Discriminatory Level 1800 – 2300 mIU/ml
  • Less invasive
  • Better view with full bladder
  • Difficult to see pregnancy of <6 wks
  • Better depth but less resolution
  • Probe usually 3-5 mHz

When Performing US:

  • Ask if the patient wants to view image, be informed of multiple gestations, and / or insert probe
  • Confirm no latex allergy; use appropriate gloves and probe cover
  • Use appropriate language to discuss US findings with patient
  • Systematically scan in 2 planes to avoid missing twins, fibroids, anomalies, etc.
    • In longitudinal view of cervix and fundus, scan side to side from ovary to ovary.
    • To get a transverse view, turn probe 90 degrees. Then scan anterior to posterior, and fundus to cervix.

         Challenger 2007

  • A limited first trimester US exam must include the following (NAF ’16):
    • Uterine scan in both transverse and longitudinal planes to confirm intrauterine pregnancy (IUP)
    • Evaluation of pregnancy number
    • Measurements to document gestational age
    • Evaluation of pregnancy landmarks, such as yolk sac or the presence or absence of fetal/embryonic cardiac activity.
  • Switch to the other probe (abdominal or vaginal) if initial scan is inadequate.

The Gestational Sac

The Gestational Sac (GS) is the first US evidence of a pregnancy, and can appear as early as 4 weeks. Until the presence of a yolk sac or embryo is seen on US, the goal of assessing an intrauterine sac is to determine if it is morphologically consistent with an early intrauterine pregnancy, or concerning for a pseudosac, which is associated with an ectopic pregnancy. After the passage of time, the GS should grow in size and show signs of development in a normal pregnancy. Absence of these sings is concerning for a non-viable pregnancy and will be reviewed below.

GS Measurement and Calculation of Gestational Age:

Measure 3 dimensions in 2 planes:

  • Longitudinal Plane: Length (L) & height (H)
  • Transverse Plane: Width (W)

 

Calculate the Mean Sac Diameter (MSD):

  • MSD = (L + W + H)/3

 

Calculate the Gestational Age (GA):

  • GA (in days) = MSD (in mm) + 30

 

Diagnosis of non-viable pregnancy:

  • Empty GS >25 mm diameter
Longitudinal view Transverse view
Images: Fjerstad, M

A normal early GS can be characterized by the FEEDS mnemonic, although meeting all criteria does not exclude the possibility of ectopic pregnancy (Fjerstad 2004). See below for more images of abnormal pregnancies, including signs of ectopic pregnancy.

Gestational Sac or Pseudosac?

Gestational Sac

Gestational Sac

Pseudosac

Pseudosac

F – Fundal (in mid to upper uterus)
E – Elliptical or round shape in 2 views
E – Eccentric to the endometrial stripe
D – Decidual reaction (surrounded by a
thickened choriodecidual reaction; appears like fluffy
white cloud or ring surrounding sac)
S – Size > 4 mm (soft criteria)
Compared to the GS, the pseudosac is more
irregular, central, smaller, and without
a decidual reaction, and can be seen with
an ectopic pregnancy. Note the “beak-shaped”
appearance of the sac, which can look similar to an
early GS, although may only meet the
F (fundal) criteria of FEEDS
Images: Fjerstad, M, Andrews, M, Gatter, M. US in Very Early Pregnancy and Management. CAPS, 2004.

Yolk Sac

The Yolk Sac (YS) is the first US finding that confirms an intrauterine pregnancy. Its presence excludes a pseudosac and confirms an intrauterine pregnancy. The YS is a round echoic ring with anechoic (dark) center seen within the GS. It appears typically at 5 ½ weeks when the MSD is 5-10 mm. The size of the YS is not generally significant, unless large and no embryonic pole.

Image from Manual Vacuum Aspiration, a presentation by PRCH & ARHP, 2000

Image from  ARHP & Physicians, 2000

The Embryo and Cardiac Activity

The embryo follows a predictable path of development and therefore can be used to date a pregnancy based on its size. The embryo appears at approximately 6 weeks and grows 1 mm per day thereafter until 12-14 weeks. After 12 weeks, fetal flexion and extension make measuring length more challenging and using the fetal biparietal diameter (BPD) is preferred. Cardiac activity appears around 6 ½ weeks.

Crown Rump Length (CRL) Measurement

  • CRL = fetal pole (in mm)
  • Long axis not including limbs or YS
  • Calculate: GA (days) = CRL + 42
Biparietal Diameter (BPD) Measurement

  • > 12 – 14 weeks
  • Inside to outside of skull
  • At the level of the thalamus
  • No nuchal or eye structures
 Images from AIUM 2013

Determining Pregnancy Viability

The following data on viability evaluated patients with desired pregnancies (Doubilet 2013).  If a pregnancy is undesired, there is no reason to delay uterine aspiration to wait for diagnosis; and a diagnostic aspiration will assist in the evaluation of a possible ectopic pregnancy (Edwards 1997).  If a pregnancy is desired, and findings are suggestive of early pregnancy loss, recheck ultrasound in 7-10 days.

 

Guidelines for Transvaginal Ultrasonographic Diagnosis of Early Pregnancy Loss in a Patients with an Intrauterine Pregnancy of Uncertain Viability* (Doubilet 2013)

US findings HIGHLY SUGGESTIVE of EPL † US findings DIAGNOSTIC of EPL
  • CRL 5-7mm and no cardiac activity
  • MSD 16-24mm and no embryo
  • MSD 13 mm or more and no YS
  • Absence of embryo with heartbeat:
    • 7-13 days after a scan that showed a
      gestational sac without a yolk sac
    • 7-10 days after a scan that showed a
      gestational sac with a yolk sac
  • Absence of embryo 6+ wks after LMP
  • Empty amnion (amnion seen adjacent to
    yolk sac with no visible embryo)
  • Enlarged yolk sac (>7mm)
  • Small gestational sac in relation to
    the size of the embryo (<5mm
    difference between MSD and CRL)
  • CRL 7+mm and no cardiac activity
  • MSD 25+mm and no embryo
  • Absence of embryo with heartbeat:
    • 2+ weeks after a scan that showed a
      gestational sac without a yolk sac
    • 11+ days after a scan that showed a
      gestational sac with a yolk sac

* Criteria are from the Society of Radiologists in Ultrasound Multispecialty Consensus Conference, October 2012.
† If a pregnancy is desired, and findings are suggestive of early pregnancy loss, recheck ultrasound in 7-10 days.

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