It is Shane's. It was pretty ovbvious. Rick came back, sex happened, then the trip to Atlanta, then they left for the CDC, stayed there a day, then went on and found the farm. All this happens in only a couple of days, 7 or 8. You don't suspect a pregnancy that fast. It takes a week for the egg even to take and hormone production to change. edit: Even blood test can detect those hormones only 6 to 9 days after impregnation, let alone over the counter urine tests.