How can it be Brody's? He was back in the US for over two weeks and we know that due to the 16 day training montage. So we can assume 3 weeks due to the heroin clearing out of his system. That makes it 12 weeks or a little less than 3 months from the bombing until when they brought Brody back into the country. Are we supposed to assume that everything that happened with Carrie in the mental hospital and with the CIA has taken place in under 3 months? And that's assuming Carrie had sex with Brody and got pregnant the day the bombing happened.
As a starting point, the last time they had sex was at the cabin the day before the memorial for the vice president. Using way too much information gathered from thinking about the events and checking Wikipedia episode entries, I've got:
1 day to the attack
58 days between the attack and Carrie giving testimony
7 days tops for Carrie in the hospital (she is initially only committed for 24 hours, then 1-3 days more plus some padding) and meeting with the lawyer as soon as she's out
1 day for adventures in finding Dana and Carrie being kidnapped
2 days for meeting Javadi, his murder spree, Saul's interrogation and delivering Javadi to the plane
72 days minimum, aka 10 weeks
At this point, the next thing we see is Saul briefing the White House and Carrie being told she's 13 weeks pregnant. Then when Brody arrives at the Iran border, Quinn says she's about 15 weeks pregnant.
That timeline puts her with Brody, with generous room to pad out some of these events where the show is less specific, but they have given a lot of actual numbers, and we have seen a lot of events just roll right into the next ones. They slept together a number of times in S2. Pick a time for conception, the timeline will be workable.