The couple, who married last June (02), have been spotted outside a natural birth clinic in London - where pals say Heather had an ultra-scan.
The pair have hinted in the past that they'd like to start a family, despite McCartney's advancing years - he will celebrate his 61st birthday in June - and 35-year-old Heather's fears health complications would prevent her from having children. The former model - who lost a leg in a motorbike accident in 1993 - lost two babies to ectopic pregnancies.
But pals say, "They are keeping their fingers crossed everything goes well for the next six months.
"Of course they are going to be worried but things are looking good and they're like any other couple waiting to become parents."
McCartney, who already has four children from his previous marriage, is said the be "ecstatic".