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 literature regarding the correlation between quantitative β-h CG titers and early intrauterine gestational sacs and embryonic structures has been made somewhat confusing by the array of reference standards used to quantify β-h CG.
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.
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.
Any health-care provider performing diagnostic ultrasound must understand the physics of diagnostic ultrasound and have thorough, supervised training.
This includes, but is not limited to, power settings, basic orientation, and proper cleaning of ultrasound probes between uses.
Intrauterine sacs embryonic structures Warren and associates described the orderly appearance of gestational sac, yolk sac, and embryo with heartbeat at a given number of days from the onset of the last menstrual period (Table 1).
With a transvaginal probe, a 2- to 3-mm gestational sac can usually be seen by 5 weeks from the last menstrual period (Fig 1A).
A yolk sac is usually seen by 6 menstrual weeks, or by the time the mean diameter of the sac has reached 10 mm (Fig 1B and Fig 1C).
A fetal pole with heart tones is typically seen by the completion of 7 menstrual weeks (Fig 1D).
For United States readers, the website, aium.org, of the American Institute of Ultrasound in Medicine will be helpful for guidelines.