Introduction

Interstitial pregnancies (IPs) occur in 2-4% of ectopic pregnancies(1). An IP  is when a gestational sac (GS)  implants in the intramural section of the fallopian tubes(1). Early diagnosis is key to prevent  catastrophic haemorrhage, which is greater in IPs due to the involvement of the myometrium and the ability to grow to large sizes(2). Diagnosis is challenging.  During ultrasound it can be difficult to differentiate from an intrauterine pregnancy and there are subtle ultrasound features to look out for. Furthermore, only 40% of women display symptoms so diagnosis may be late (3). 

Case Report

A 40-year-old multiparous woman (3 previous SVDs) had her routine 12 week dating scan  which showed a yolk sac and gestational sac of unknown viability at Frimley Park Hospital, Surrey UK. The pregnancy was considered to be intrauterine in location at that time. She was referred to our Early Pregnancy Unit in December 2022 to confirm viability.  She had no symptoms, besides a one day history of PV spotting that had resolved 2 days prior to presenting to us.  She had no risk factors for ectopic pregnancies.

 

The ultrasound scan performed two weeks later, showed an empty uterine cavity (picture a)  and a highly vascular gestation sac (picture c)  measuring 45 x 51x 47mm with a very thin myometrium in the right side of uterus (picture b). The sonographer suspected an interstitial ectopic and escalated to EPU consultant. The interstitialectopic diagnosis was confirmed by 2-D as well as 3-D scan by the consultant.

 

She was offered both medical and surgical management, with the wish to avoid surgery she opted for IM methotrexate. Her BHCG trended down immediately:   14,145 (day 0), 5,220 (day 4), 2,577 (day 7) and her latest, 5 (week 10). She remained symptom free and was managed exclusively as an outpatient with weekly follow up, from 7 days onwards. Only one dose of methotrexate was needed. 

Ultrasound features suggestive an an interstitial ectopic (4)(5)