Rod Stewart and his pregnant wife Penny Lancaster have revealed how they were forced to turn to IVF after a long battle to have another child.
The couple had tried for two years to give their son Alastair, four, a brother or sister.
Speaking for the first time about their struggle, they told how they saw fertility experts in both America and the UK.
Miss Lancaster, 39, said she had cut fish out of her diet after doctors discovered very high levels of mercury in her body. She then became pregnant on her third round of IVF.
She insisted the gruelling procedure was worth it just to see Alastair's joy when she broke the news to him. 'Our son's happiness made all the IVF treatments worthwhile,' she said.
The new addition to the Stewart clan will arrive just weeks after the singer celebrates his 66th birthday – and shortly before his wife turns 40.
The couple described their 'emotional rollercoaster' since being told they could not conceive naturally.
Miss Lancaster was told the mercury levels in her body were a result of her love of fish. Her preference for a high-protein, low-fat seafood-based diet may explain the levels.
She said: 'A homeopathic doctor told me my mercury levels were really high, off the scale. I do love fish, and I thought it was healthy without understanding the high mercury levels that fish like tuna, swordfish and halibut can contain. My doctor said it was unhealthy to have so much mercury in my body, and also for conceiving babies. After two months of not eating fish, my mercury levels dropped by half, which is scary - I had no idea what the fish had been doing to my body.'
Excessive mercury can be dangerous for unborn children. Experts recommend that women who are pregnant or trying to conceive limit themselves to two portions of oily fish a week.
Recalling the moment she learned of her pregnancy, Miss Lancaster said: 'I couldn't believe it. I burst into tears. I called Rod when I was still sobbing. Rod said, "I'm in the middle of a restaurant in Russia, crying". I kept pinching myself. Neither of us could believe it.'
Stewart said: 'IVF can be stressful, especially for the woman with all the hormone injections and procedures. I said to Penny, "don't worry, we'll just keep trying".'
Stewart has already fathered seven children. His eldest-daughter Sarah Streeter is 46, and was conceived when the singer was just 18. His second-eldest Kimberley, 30, and Sean, 29, are from his marriage to first wife Alana.
Stewart went on to have another daughter, Ruby, 23, with girlfriend Kelly Emberg. He then had Renee, 18, and son Liam, 15, with his ex-wife, model Rachel Hunter. His new baby is due in mid-March.
Miss Lancaster insisted her husband's age was not an issue, saying: 'All the tests he's had show he's fitter than the average 45-year-old.'
Stewart added: 'I keep fit and healthy and, with a young family, there's no holding me back.'
Despite this, Stewart has said that this baby will be his last. He said: 'I'm very happy but it will be the last. I can't have any more kids, otherwise I'll be touring until I'm 90.'
The full interview with the couple appears in the latest edition of Hello!