Ultrasound criteria
A review article in The New England Journal of Medicine based on a consensus meeting of the Society of Radiologists in Ultrasound in America (SRU) has suggested that miscarriage should be diagnosed only if any of the following criteria are met upon ultrasonography visualization:

Classification
A threatened miscarriage is any bleeding during the first half of pregnancy. At investigation it may be found that the fetus remains viable and the pregnancy continues without further problems.An anembryonic pregnancy (also called an "empty sac" or "blighted ovum") is a condition where the gestational sac develops normally, while the embryonic part of the pregnancy is either absent or stops growing very early. This accounts for approximately half of miscarriages. All other miscarriages are classified as embryonic miscarriages, meaning that there is an embryo present in the gestational sac. Half of embryonic miscarriages have aneuploidy (an abnormal number of chromosomes).An inevitable miscarriage occurs when the cervix has already dilated, but the fetus has yet to be expelled. This usually will progress to a complete miscarriage. The fetus may or may not have cardiac activity. A complete miscarriage is when all products of conception have been expelled; these may include the trophoblast, chorionic villi, gestational sac, yolk sac, and fetal pole (embryo); or later in pregnancy the fetus, umbilical cord, placenta, amniotic fluid, and amniotic membrane. The presence of a pregnancy test that is still positive, as well as an empty uterus upon transvaginal ultrasonography, does, however, fulfil the definition of pregnancy of unknown location.
Contraindications
Combining the drug with pimozide or ergot alkaloids (such as ergotamine or methylergometrine) is contraindicated because these drugs are metabolized by the liver enzyme CYP3A4, and letermovir inhibits this enzyme. In people who also take ciclosporin, which increases letermovir concentrations in the body, combination with the cholesterol lowering drugs simvastatin and pitavastatin is also contraindicated. In Canada, this also applies to bosentan, lovastatin and rosuvastatin; and in the EU, to dabigatran, atorvastatin, and rosuvastatin. Adverse effects
Side effects from the use of letermovir are uncommon, but gastrointestinal symptoms such as gastritis and nausea may occur, as can dyspnea (difficulties breathing) and hepatitis. In general, side effects of the drug are comparable to those under placebo treatment. Overdose
In studies, giving the threefold therapeutic dose for 14 days resulted in no additional adverse effects. It is unknown whether the substance can be removed from the system by hemodialysis. Pharmacology
Mechanism of action
Letermovir is a viral terminase inhibitor. It specifically inhibits the CMV viral terminase complex which is encoded by the CMV genes UL56, UL51 and UL89. This inhibition has the effect of preventing cleavage of CMV DNA concatamers, resulting in long uncleaved DNA and noninfectious viral particles. Letermovir is only active against CMV and has no effect on other herpesviruses. Pharmacokinetics
Letermovir is quickly absorbed from the gut, reaching its highest concentrations in the blood plasma after 1.5 to 3 hours. Its bioavailability is estimated to be 37%. Ciclosporin increases this bioavailability to about 85%. When in the bloodstream, the substance is almost completely (98.2%) bound to plasma  proteins.