The phenomenon is called "cryptic pregnancy" or pregnancy denial, and occurs in approximately 1 in 2,500 pregnancies. These women find out about their pregnancies usually at the onset of the fetal quickening, i.e. at 20 weeks in primipara, and 16 weeks in multipara.

Contributing factors include:

(1) inaccurate (false-negative) pregnancy tests;

(2) pregnancy-related weight gain is less noticeable in overweight women;

(3) minimal pregnacy related symptoms (morning sickness, fagitue, soreness, food craving, etc)

(4) some women continue having periods throughout the pregnancy (in addition, those women who had irregular menstrual cycles may confuse the spotting with an irregular pattern);

(5) obese women, with thick abdominal wall, may poorly feel the fetal kicks;

(6) if the placenta is located on the front uterine wall, this could provide extra cushioning and may prevent the woman from feeling the pitter-patter of tiny feet;

(7) oligohydramnios (caused by multiple factors, most primarily, by infections) would result in fetal passiveness, and the woman may not feel the fetal movement;

(8) women (younger 42) formerly treated for infertility, may confuse the pregnancy with premature ovarian failure;

(9) older women (over 45) may confuse the pregnancy with menopause;

(10) women with Axis-1 group mental illnesses (bipolar disorder-both phases, schizophrenia-negative phase) may lack in coping and attachment skills to address the pregnancy and therefore may deny it;

(11) there are several medical conditions that may reduce the woman's perception of pregnancy.