pregnancy may be detected either based on symptoms by the woman herself, or by using pregnancy tests. However, an important condition with serious health implications that is quite common is the denial of pregnancy by the pregnant woman. About 1 in 475 denials will last until around the 20th week of pregnancy. The proportion of cases of denial, persisting until delivery is about 1 in 2500. Conversely, some non-pregnant women have a very strong belief that they are pregnant along with some of the physical changes. This condition is known as a false pregnancy. Physical signs Most pregnant women experience a number of symptoms, which can signify pregnancy. A number of early medical signs are associated with pregnancy. These signs include: the presence of human chorionic gonadotropin (hCG) in the blood and urine missed menstrual period implantation bleeding that occurs at implantation of the embryo in the uterus during the third or fourth week after last menstrual period increased basal body temperature sustained for over two weeks after ovulation Chadwick's sign (darkening of the cervix, vagina, and vulva) Goodell's sign (softening of the vaginal portion of the cervix) Hegar's sign (softening of the uterine isthmus) Pigmentation of the linea alba – linea nigra, (darkening of the skin in a midline of the abdomen, resulting from hormonal changes, usually appearing around the middle of pregnancy). Darkening of the nipples and areolas due to an increase in hormones. Biomarkers Pregnancy detection can be accomplished using one or more various pregnancy tests, which detect hormones generated by the newly formed placenta, serving as biomarkers of pregnancy. Blood and urine tests can detect pregnancy 12 days after implantation. Blood pregnancy tests are more sensitive than urine tests (giving fewer false negatives). Home pregnancy tests are urine tests, and normally detect a pregnancy 12 to 15 days after fertilization. A quantitative blood test can determine approximately the date the embryo was conceived because hCG doubles every 36 to 48 hours. A single test of progesterone levels can also help determine how likely a fetus will survive in those with a threatened miscarriage (bleeding in early pregnancy). Ultrasound Obstetric ultrasonography can detect fetal abnormalities, detect multiple pregnancies, and improve gestational dating at 24 weeks. The resultant estimated gestational age and due date of the fetus are slightly more accurate than methods based on last menstrual period. Ultrasound is used to measure the nuchal fold in order