According to Helen Cheyne, professor of maternal and infant health research at the University of Stirling, "it is not known" how a cryptic pregnancy like Bryony's can happen.
She said: "It's actually one of these rather ill-understood phenomena [and] I think it's very difficult for people to understand - how could a woman not know that they were pregnant?
"We kind of have this intuitive notion that 'of course a woman will know she's pregnant as soon as she is' but historically, that wouldn't have been the case."
Prof Cheyne explained before modern methods of testing for pregnancy, women relied on more subtle signs that indicated they were expecting like feeling the baby move, which typically doesn't happen for first-time mothers until about 20 weeks.
She said there was no medical definition of a cryptic pregnancy and that there is also an absence of data collected on them.
"[They are] rare, but not exceptionally rare," she said.
"Most of my midwife colleagues had either come across it, or had heard of it, around their clinical practice."
While working as a midwife in the early 1980s, Prof Cheyne met a woman in her mid-40s with other children who claimed to have had a cryptic pregnancy.
She said the woman assumed she was entering the menopause, which was why she felt she was putting on weight and had stopped having periods.
Prof Cheyne said as few pregnancies were cryptic, it is not something women needed to be concerned about.