Cardiac rhabdomyoma (CR) is the cardiac tumour most commonly diagnosed in utero. Eighty percent of CRs are associated with tuberous sclerosis (TS). TS is a rare multi-system disease, with autosomal dominant genetic transmission. If the parents of an affected child do not have features of TS, then either one parent is mosaic for the TS gene mutation or the affected child is the result of a de novo germline mutation. We present a case of a dizygotic twin pregnancy complicated by CRs in both fetuses at 24 weeks. Twin A died in utero at 28 weeks. Preterm labour and delivery of twin B occurred at 33 weeks. Twin B had multiple small CRs and a large apical CR. At six weeks after delivery, the CRs had disappeared or reduced in size. Regression in the third trimester or postnatally is the natural course of CRs. Molecular testing for TS identified two variants in the TSC2 gene. The parents were clinically unaffected; however, the father was subsequently found on an MRI of the head to have cortical tubers, and he was found to carry the pathogenic TSC2 mutation. Since dizygotic twin pregnancy is akin to two consecutive pregnancies, the etiology in our case is due to one parent having subclinical TS. To the best of our knowledge, this is the first such case to be reported.