Not knowing your 13yo pregnant

My cousin cousin daughter gave birth. I don't see my cousin much we talk a lot on the phone. She told me she didn't know her daughter was pregnant until one day her daughter went to use the bathroom and went in labor and her daughter also didn't know she was pregnant until then. I asked how could she not know she said her belly never got big.

I know its possible but what do you think? Do you think my cousins lying or she's just protecting her daughter from negativity??