The couple had tried for two years to give their son Alastair a brother or sister but sought medical help after efforts to conceive naturally failed.
They told this week's Hello! how they tried fertility experts in both America and the UK. Lancaster, who posed for pictures with Rod at their LA mansion for the magazine, said Alastair was thrilled at the prospect of a sibling and added: "Our son's happiness made all the IVF treatments worthwhile."
The couple have been on an "emotional rollercoaster" since being told they could not conceive naturally. And Lancaster was told mercury levels in her body were "off the scale" as a result of her love of fish. She cut out seafood and after a third IVF cycle their dreams came true.
Recalling the moment she learned of her pregnancy, she told Hello!: "I couldn't believe it. I burst into tears."
The new baby is due in mid-March when Rod will be 66.