(Cover) - EN Showbiz - Rod Stewarts wife Penny Lancaster has revealed killer cells may have killed her unborn babys twin.

The couple announced they are expecting a baby last month, which will be the rockers eighth child. The 65-year-old and his 39-year-old wife already have four-year-old son Alastair together and have revealed fertility problems caused them to use in vitro fertilisation (IVF) to help Penny fall pregnant this time round. Penny has now spoken about the treatment, explaining doctors think she may have been pregnant with more than one baby.

The star has been treated for an underactive thyroid gland for years, and killer cells linked to the condition may have attacked the foetus.

I was diagnosed with an underactive thyroid in my 20s. My antibodies eat away at it. I've been on drugs for the condition for years, British newspaper The Sun quotes her as saying. "When I got the positive pregnancy test I found out the killer cells were becoming aggressive. Similar to the way they attacked my thyroid, they might attack the embryos. The doctors put me on steroids. They came to the conclusion there might have been more than one baby, but the killer cells had got to one of them.

Penny explained she never thought she would struggle to conceive following the birth of Alastair. Before having him she suffered a miscarriage, but did not expect to have more problems.

When we first decided to have children I fell pregnant and sadly had a miscarriage, she said. "The very next month I fell pregnant with Alastair. Everything went smoothly and I had a natural birth, so there was no reason to assume we'd have difficulty conceiving again

Penny became pregnant after three rounds of IVF, with the baby due in early 2011. (C) Cover Media