Background
Epithelioid and placental site trophoblastic tumours are rare gestational malignancies which have little detailed information on their population incidence or risk related to maternal age.

Methods
We performed a retrospective UK national population-based study examining all of the cases registered between 2000 and 2020 using the databases at the UK’s two gestational trophoblastic treatment centres at Charing Cross Hospital in London and Weston Park Hospital in Sheffield. The data obtained was compared with the contemporary UK birth and pregnancy statistics.

Results
Over the 21-year study period, there were 132 cases of ETT or PSST. PSTT comprised 57% of the cases, 30% were ETT and 13% had mixed pathology. The combined incidence of ETT and PSTT was 1:118,736 relative to live births and 1:150,872 compared to total viable conceptions. For women aged under 20 the incidence relative to live births was 1:412,488, increasing to 1:188,292 for women 30–34 years, and 1:1,426 for women aged 45 and above. The median interval from the antecedent pregnancy to the time of diagnosis was 15 months (0–288) for the PSTT patients compared to 24 months (0–336) for patients with a diagnosis of ETT.

Conclusions
ETT and PSTT are both rare diagnoses with little detailed information on their demographics. The data in this study indicates a wide range in the interval from the antecedent pregnancy to diagnosis and confirms a close relationship between increasing incidence and rising maternal age.

PLAIN LANGUAGE SUMMARY
Epithelioid and placental site trophoblastic tumours are two similar but rare tumours arising from the cells of pregnancy. Due to their rarity there is little accurate information on the risk of having these diagnoses and how that risk may change with increasing maternal age. In a 21-year UK wide national study we found a total of 132 cases from a total of approximately 19.9 million pregnancies giving an overall risk of 1:150,872.

The risk increased appreciably with rising maternal age increasing from 1:526,349 in women aged under 20 to 1:10,343 for women 45 and over. Despite the rarity the large majority of patients are cured with modern therapy.