I apologize if this is a dumb question, but I can't seem to remember the answer. You know when sometimes a person is scheduled for surgery and so their type and screen expiration date is extended to the surgery date plus 2 or 3 days. One of the requirements in order to extend the expiration date is that the patient has no transfusions or pregnancies in the last 3 months. What is so special about 3 months? I know this is probably a dumb question. Thank you in advance. 

I believe it is because the average time for the immune system to mount a primary response is 90 days. Therefore, if they have had exposure within that time, detection of a clinically significant antibody may not happen. Of course, transfusion and pregnancy are significant exposures that would elicit a primary immune response

The three months was chosen following a paper written by Laine EP, Leger RM, Arndt PA, Calhoun L, Garratty G, Petz LD. (In vitro studies of the impact of transfusion on the detection of alloantibodies after autoadsorption.  Transfusion 2000; 40 1384-1387.  DOI: 10.1046/j.1537-2995.2000.40111384.x.) that showed that red cells that had been transfused (or entered the circulation  via a feto-maternal haemorrhage could adsorb out weak alloantibodies for up to three months in a patient with AIHA.  This in vivo adsorption would, of course, also apply to individuals who did not have AIHA, but could lead to a secondary stimulation, leading to a stronger antibody (higher titre and higher concentration per mL of plasma), if the alloantibody was "missed" in the antibody screen and/or cross-match, particularly as it is unlikely that the full phenotype of the transfused (or foetal) red cells would be known.