I am having more then one child with hypoplastic left heart syndrome, what could cause this?

Can be familial: Hypoplastic left heart syndrome, in some cases, appears to have a familial (genetic) component. There are various genes that have been suggested to have influence over cardiac development where mutations in one of those genes may result in hlhs. Additionally, there are families where bicuspid aortic valve and hlhs (and other left-sided obstructive lesions) have related inheritance.