Rod Stewart and Penny Lancaster had to undergo in-vitro fertilisation (IVF) to have another child.
The couple - who already have a four-year-old son, Alastair, together - were desperate to give their little boy a sibling but after trying unsuccessfully for two years, they decided to seek help from fertility experts.
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.
After speaking to doctors, Penny, 39, cut fish out of her diet after being told she had very high levels of mercury in her body and after a third attempt of IVF, she conceived.
Penny 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."
Speaking about the moment she discovered she was pregnant, Penny admitted both she and the 'Maggie May' hitmaker broke down in tears.
She recalled: "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."
Despite the gruelling IVF battle, Penny says seeing the joy on Alastair's face when he was told he was going to be a big brother made it all worthwhile.
She told Britain's Hello! magazine: "Our son's happiness made all the IVF treatments worthwhile."
Rod - who has six other children from previous relationships - admitted the process was quite stressful but they were determined to conceive.
The 65-year-old rocker 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.' "