It takes advantage of the differential resistance of foetal hemoglobin to acid. A standard blood smear is prepared from the mothers blood, and exposed to an acid bath. This removes adult hemoglobin, but not foetal hemoglobin, from the red blood cells. Subsequent staining, using Shepards method, makes fetal cells (containing foetal hemoglobin) appear rose-pink in color, while adult red blood cells are only seen as "ghosts". 2000 cells  are counted under the microscope and a percentage of foetal to maternal cells is calculated. Fetal-maternal haemorrhage can also be diagnosed by flow cytometry, using anti-foetal hemoglobin antibodies (anti-HbF). Treatment
If ongoing and rapid haemorrhage is occurring then immediate delivery of the foetus may be indicated if the fetus is sufficiently developed. If the  haemorrhage has already occurred and now stopped, an inutero transfusion of red cells to the foetus may be recommended. References


== External links ==
Discusses relationship between hymen and concept of virginity. Putting tampon in painlessly Radiology (US - ultrasound) of Hydrocolpos
Evaluating the Child for Sexual Abuse at the American Family Physician
My Corona: The Anatomy Formerly Known as the Hymen & the Myths That Surround It Scarleteen, Sex education for the real world
The Hymen Myth
Vaginal Corona