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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.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.Am J Obstet Gynecol 17, 1989) Fig 1A A very early, 3-mm mean diameter intrauterine gestational sac at 5 weeks postmenstruation Fig 1B A typical yolk sac.The role of the early anatomic survey, when free DNA screening for chromosomal anomalies is being utilized, remains to be determined.

Failure to understand the limitations of diagnostic ultrasound or inadequate training of physicians in this technique can result in grave complications for the patient and liability for health-care providers.

The potential benefits of a subsequent ultrasound examination at 12–14 weeks from the LMP include: Similarly the standard of care for performing routine ultrasound at 12–14 weeks' gestation from the LMP varies from country to country.

The disadvantages are cost, decreased sensitivity for major anomalies compared to the 20 week exam, and marked increase in training requirements for providers.

Alternatively for international readers, the website, isuog.org, of The International Society of Ultrasound in Obstetrics and Gynecology should be searched for similar guidelines.

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).