A.T. Robertson Commentary Romans 9:10

"And not only so; but Rebecca also having conceived by one, [even] by our father Isaac--" — Romans 9:10 (ASV)

Having conceived of one (εξ ενος κοιτην εχουσα). By metonomy with cause for the effect we have this peculiar idiom (κοιτη being bed, marriage bed), "having a marriage bed from one" husband. One father and twins.