Anembryonic pregnancy

Anembryonic pregnancy is a form of a failed early pregnancy, where a gestational sac develops, but the embryo does not form. 

Terminology

The term blighted ovum is synonymous with this but is falling out of favor and is best avoided. 

Clinical presentation

The patient may be asymptomatic, presenting for an early pregnancy ultrasound. Alternatively, she may present with vaginal bleeding in early pregnancy. Due to falling beta-hCG levels, the clinical signs of pregnancy tend to subside.

Pathology

In anembryonic pregnancy, a blastocyst is formed from a fertilised ovum, but the fetal pole/embryo never develops, though histologically some fetal material can be demonstrated in most cases.

Beta-hCG is formed due to the invasion of the endometrium by the syncytiotrophoblast, and as a result, there is a positive pregnancy test and clinical signs of pregnancy are present. 

Radiographic features

Ultrasound

An anembryonic pregnancy may be diagnosed:

• when there is no embryo seen on endovaginal scanning in a gestational sac with mean sac diameter (MSD) ≥25 mm ⁴

or

• there is no embryo on follow-up endovaginal scan ¹⁰ 

  • ≥11 days after scan showing gestational sac with yolk sac, but no embryo, or

  • ≥2 weeks (14 days) after a scan showing gestational sac without yolk sac or embryo ^ref

Assessment of interval MSD growth has been shown to be insufficiently accurate in the diagnosis of anembryonic pregnancy, due to an overlap of gestational sac growth rates of viable and non-viable pregnancies. 

Other ancillary features have been described, and may be considered poor prognostic factors, but do not contribute to the formal diagnosis of a failed pregnancy. These include: 

• absent yolk sac when MSD >8 mm on transvaginal ultrasound (TVUS)

• poor decidual reaction: often <2 mm

• irregular gestational sac shape

• abnormally low sac position

Differential diagnosis

Conditions that cause the appearance of an empty gestational sac include:

• early pregnancy (intrauterine) 

• pseudogestational sac

• gestational trophoblastic disease: especially 1^st-trimester molar pregnancies ⁵