Anembryonic pregnancy

Anembryonic pregnancy is a form of a failed early pregnancy, where a gestational sac develops, but the embryo does not form. The term blighted ovum is synonymous with this, but is falling out of favour and is best avoided. 

The patient may be asymptomatic, presenting for an early pregnancy ultrasound. Alternatively, she may present with vaginal bleeding in early pregnancy. Due to falling 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.

Human chorionic gonadotropin (beta-hCG) is formed due to invasion of endometrium by the syncytiotrophoblast, and as a result there is a positive pregnancy test and clinical signs of pregnancy are present. 

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 hCG levels, the clinical signs of pregnancy tend to subside.

Radiographic features

Ultrasound

An anembryonic pregnancy may be diagnosed when there is no fetal pole identified on endovaginal scanning⁴, and:

• the size of the gestational sac is such that a fetal pole should be seen: MSD ≥25 ≥25 mm on TVS (by RCOG criteria)

Or

• there is little or no growth of the gestational sac between interval scans
  • normally the MSD should increase by 1 mm mm per day
  • if the MSD is too small to determine the status of the fetus on the initial ultrasound, a follow up scan in 10-14 days should differentiate early pregnancy from a failed pregnancy (see: pregnancy of uncertain viability)

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;8 mm (on TVS)
• poor decidual reaction: often <2;2 mm
• irregular gestational sac shape
• abnormally low sac position

Differential diagnosis

Conditions that cause an empty gestational sac include:

• early pregnancy (intrauterine) 
• pseudogestational sac
• gestational trophoblastic disease: especially 1^st trimester molar pregnancies ⁵