Having concealed pregnancies or unknown pregnancies is actually not all that uncommon. The literature says that about one in 500 pregnancies, the woman appears either late in pregnancy or as late as being in labour.
This is more common among busy women, those who may already be overweight or those approaching menopause. It tends to happen in menopausal women and younger busy women because they are not expecting to be pregnant and won’t be looking for signs.
There are risks, particularly to the developing brain of the baby, and there is a syndrome called foetal alcohol syndrome where these babies are born with brain injury and other physical problems because mums have drunk alcohol across the pregnancy.