Ultrasound dating of pregnancy
These “pseudogestational sacs” can lead to a missed diagnosis of ectopic pregnancy (Figures 2A–2D).
Normal early gestational sacs are seen eccentrically placed in the decidua, adjacent to the echogenic central stripe (Figure 1A). A review of the literature by Richardson et al., from 2015, found the intradecidual sign was 66% sensitive and 100% specific in identifying an intrauterine pregnancy.
Suffice it to say that the presently used Third International Standard and Fourth International Standard, used by most companies marketing β-h CG kits, corresponds roughly to the First International Reference Preparation.
The practitioner must understand that these β-h CG levels are guidelines for the appearance of these structures and are not 100% reliable or accurate. Table 1 The appearance of early gestational structures LMP, last menstrual period; FHTs, positive fetal heart tones(Warren WB, Timor-Trisch I, Peisner DB et al: Dating the early pregnancy by sequential appearance of embryonic structures.
Today's obstetrician gynecologist is required to interpret and in many cases perform ultrasound scans in the first trimester.
In fact, certification of residency programs in many countries requires documentation of adequate exposure to and training in the evaluation of first-trimester ultrasound.
Fertil Steril 8, 1988) Discriminatory zones The generally accepted β-h CG discriminatory zone of 2000m IU/ml for the appearance of an intrauterine gestational sac is not 100% reliable.
Recent papers by Doubilet and Benson and Ko and Cheung have provided examples of women with titers greater than 2000 m IU/ml, where transvaginal ultrasound did not identify a early intrauterine sac, and a subsequent ultrasound confirmed a viable pregnancy.
Alternatively for international readers, the website, isuog.org, of The International Society of Ultrasound in Obstetrics and Gynecology should be searched for similar guidelines.
These exceptions to the previously accepted cut-offs have important implications in the management of threatened abortion, missed abortion, and ectopic pregnancy.
Pseudo-sacs Considerable caution must be exercised not to mistake collections of fluid within the endometrial cavity for early gestational sacs.
We also rescan the patient in 7–10 days to document the appearance of the yolk sac.
Fig 2A An eccentrically placed intrauterine gestational sac 6 weeks postmenstruation (arrow) Fig 2B In contrast, a pseudosac (arrow) in a patient with ectopic pregnancy representing a collection of blood or fluid collected within the endometrial cavity.