According to u.s., i am due dec 22, 2015. which is the more likely date of conception, march 31st or april 4th?

Too close to tell...: In theory, calculating date of conception is just pure math. However, there's a range of probable vs possible dates given lifespan of egg & sperm. 2 calculators (http://goo.gl/ULNOoE & http://goo.gl/YKIZjS) included both March 31st & April 4th while 3rd (http://goo.gl/9pe8Oe) only included former. None of 3 stated that latter alone was date of conception w/o including former. Hope that makes sense