First, I counted the days.
*Buffy's party is day zero.
*She wakes up with naked Willow on day one and later meets Spike and the loan collector.
*On day one or two she is caught by the cops with a stake.
*On that same day she escapes and the next meets up with the Siphon.
*She sleeps over with the Siphon on day two or three.
*She does battle with the Siphon with Dowling, Koh, and Spike's help on day three or four.
*On day three or four, Anaheed and Tumble find her weapons stash.
*Anaheed and Tumble are so eager to talk about her dangerous lifestyle, they harass her in the bathroom soon thereafter. Since it's such a priority to them, I can't imagine they've waited more than 48 hours after they found the weapons, so it's day four to six.
*Buffy is taking a pregnancy test in that bathroom and gets a little blue plus on day four to six.
So I conclude that Buffy is confirmed positive on the pregnancy test somewhere between four and six days after the party.
We've all had this conversation:
Paramour: Owen, I'm pregnant.
Owen: Really? How do you know?
P: It's my cycle. It hasn't started yet.
O: Are you sure?
P: Of course I'm sure! It's my body.
O: I mean, are you sure you're overdue.
P: I think so. I must be. It feels overdue.
O: Have you tried a pregnancy test?
P: Those don't work until you've been pregnant for two weeks.
O: So let's just not worry about it yet.
P: Owen, I can't be pregnant. I'm only [16,17,18,19,21,23,36,39,40] years old, we're not married, my father doesn't like you, and you don't have a job.
I always enjoy that conversation because it's a strong predictor of hot, intense panic sex.
I find that the ladies often just aren't counting right. One partner of mine would panic each and every month because she was sure her cycle should be exactly 28 days. As a rational man I keep my mind like Abed's notebook and I always know the exact day of my girlfriends' cycles and their likely mood swings. It's a survival mechanism. That 28-day girl cycled like clockwork, always 31.25 days. That made for three excellent days of panic sex each month. You might suggest that I should have relieved her of the worry, but would you really try to tell your girlfriend that you knew her cycles better than she did?
It sure beats the alternative conversation:
P: Owen, you need to move out of my house.
O: What? I like it here.
P: It's been four months, you still don't have a job, and I'm pregnant.
O: You're pregnant? That's great, but I definitely need to stay around for the baby.
P: So it can have a daddy lying around on my sofa, drinking my beer, playing video games, and freeloading? I don't think so.
O: That's not all I do.
P: It was great that one week when you organized the recall campaign against our asshole state representative, but that was two months ago and it never got you a paying job. I've only been sleeping with you because of the way you told off my [boss,graduate advisor,father] and how sexy you look on your bike with that leather coat. But the rebel without a cause thing is over. It hardly makes a marriage.
O: But we're having a baby together.
P: I'm having a baby; you're moving out. Pack up your things and call your mother and see if she'll take you in. Again.
I've had that one a few times, too, and it never leads to hot panic sex.
Anyway, the point here is that it takes about two weeks for a pregnancy test to work. Even the sensitive ones your doctor can do don't work in less than a week and a half.
Thus I'm forced into one of two conclusions:
1. Buffy was pregnant before her party. Maybe she and Spike got frisky on the fire escape. Maybe Xander and Buffy got it going on Xander and Dawn's bed and that's why they chatted guiltily before the party and Buffy had to move out so suddenly and that's why Dawn was upset with Xander and wouldn't let Buffy stay over (lots of evidence for this, actually). Maybe it was an immaculate conception. In any case, the baby isn't just a Dark Horse punishment for girls who wear skirts and drink, because it happened before S9 started.
2. The comic writers are boys. The boys in question have never had a serious girlfriend and never inquired into how human females operate. In fact, these boys aren't really interested in women except as giant-breasted icons of inky desire unthreateningly confined to the inanimate page.
Could be either one, really.