For One Night by Penny Jordan

Desolated by the loss of a close friend, Diana had fallen into the arms of gorgeous stranger. It had a been a heart-stopping night of searing passion which she would never forget...especially because she's now pregnant! Making a new life for herself and her baby, Diana moves to a new town. But fate has obviously a bigger plan... The baby's father lives there too! And successful billionaire, Marcus Simpson, wants way more than just one night...

Originally published in 1987.