"The buzz in Jen's inner circle is that she's nearly three months along but doesn't want the news to come out until she knows for sure that the baby will be okay," a source said.
Aniston has been dating Justin since 2011 and was reportedly thinking about her chances of having a baby six months into their relationship.
"Jen started having fertility treatments six months into the relationship. She knew the older she got, the harder it would be to get pregnant, so she had her eggs frozen.
"With regular attempts at in vitro fertilisation, she was hoping to get lucky, but she didn't dare think about it too much because of the chance for disappointment," the source added.