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The mother is sedated in a supine position.
A 20-guage needle is passed through the anterior abdominal wall and, under ultrasound imaging, a placental vessel is sampled about 1 cm from the site of cord insertion.
The safe volume of blood to remove increases with gestational age. By the third trimester 3 ml is the upper limit.
The risk of the procedure is apparently not influenced by the position of the placenta.
Rhesus negative mothers should receive 250-500 units of anti-D immunoglobulin i.m. after the procedure.