There is a whole TV show on this topic and apparently there are enough women who didn't know they were pregnant to fill the show week after week. Skeptics secretly wonder whether this is, actually, possible. Did these women truly have no idea they were expecting a baby, or did they just hide their pregnancy (from others, or themselves) for whatever reason?
It is relatively uncommon for a woman to not realize that she was pregnant, but it does happen. I am very certain there are "hoax" cases where the person in question did realize she was pregnant but chose not to tell others as well teenage mothers, or those who got pregnant through an extramarital affair come to mind but some of these cases are indeed genuine. Don't these women have any pregnancy signs and symptoms?
They might, but they may not realize that these symptoms resulted from a pregnancy. Women who already have no periods or highly irregular menstrual cycles are more prone to get pregnant and not realize it. This can happen in cases of PCOS or other conditions that cause irregular periods, when a woman is very close to the menopause, or when she is still breastfeeding her older baby and relying in that to stop her form getting pregnant. All that is required for a woman not to know she is expecting a baby is her believing a pregnancy is impossible. Many of the women on the TLC show were on birth control when they conceived.
Those who are overweight may not notice any weight gain or changes in their bodies, and this can even happen to normal-weight women. Any unusual symptoms can easily be attributed to other things if there are no changes in weight and menstrual periods. Women who had no idea they were pregnant complained of stomach aches, depression, and fatigue. Admittedly, these thing can happen to anyone who is not pregnant.