Or the woman who stole her grandchild and pretended it was her own baby, or did the daughter steal her mother's baby and pretend it was hers? I can't remember.
Okay:
Caitlin (spoiled but sweet and naïve daughter of Sunset Beach's wealthiest and most powerful couple) met and fell in love with reformed jewel thief Cole, got engaged and pregnant.
Cole had a one-night stand with Caitlin's mother Olivia right before he met Caitlin, without knowing who she was, which resulted in Olivia also becoming pregnant. So mother and daughter were pregnant by the same man at the same time.
Olivia and her evil husband Gregory didn't want Caitlin marrying a jewel thief so Gregory came up with an idea that Olivia would pretend she was pregnant (he didn't know then that she really was pregnant and maybe by Cole!), wait for Caitlin to go into labour, drug her, then when she woke up they'd tell her that her baby died, and pretend her baby was Olivia's baby. Later Olivia reveale
Caitlin found out about the plan and drove off in despair, crashing her car and miscarrying, and was later told the crash had rendered her infertile. She continued to pretend to be pregnant while desperately trying to illegally adopt a baby to pass off as her own, because she thought Cole would dump her for being infertile.
She asked Annie (who wanted to break up Olivia and Gregory's marriage because her dad's will said she'd only receive her inheritance if she could get Gregory to marry her by a certain date) to help her get a baby. Annie finds out about Gregory and Olivia's plot and is so horrified she decides to do to them, what they'd planned to do to their own daughter - with the added benefit that losing a child will break them up (Olivia and Gregory broke up once before because Olivia's alcoholism caused her to miscarry, so Annie thinks a second miscarriage/stillbirth will break them up again).
Annie steals Olivia's baby and gives it to Caitlin, pretending it's the child of a young unmarried friend of hers who gave the baby up for adoption. So Caitlin believes she's raising some random teen mum's kid that was consensually adopted and passes off her "adopted" child as her own, without knowing the baby she adopted was actually her own presumed dead half-brother/stepson.
This is all from memory, btw!