Very dark very quick lines
I get very dark lines quickly too, and with almost all of my 11 pregnancies I start getting positives at 8dpo in the evening (so just barely even 3 weeks!). I haven't ever had twins, though, but we've suspected them multiple times.
The quickest, darkest very early line I ever had was at about 6dpo (sure of dates) and it was DARK, like 9-12dpo dark, and it was an ectopic pregnancy. So if I run into a situation like that again I will definitely be on my guard.
My peestick came up positive & DARK before it even got to the control window. ?!!!? That was a generic "plus sign" test. This was Sunday, day 28 of my usual 25 day cycle.
Monday, a cheapy dipstick test at the clinic, which barely showed a positive line at all.
Tuesday, another cheapy-looking dipstick test at my OB/GYN's office - a very faint line & took forever to show up.
21wk with a single girl in the womb. :) I think different tests just have different thresholds. :)
I don't think I would read into how dark the lines are on a positive pregnancy test. The fact is that if there is a positive, it means positive, no matter how dark the line is. Some women produce different amounts of hCG (which is the hormone detected in pregnancy tests).
It is true that with twins, you will have higher levels of hCG. But that really depends on each person.
I am now 20 weeks pregnant. When I first got pregnant (this time) I took a pregnancy test at what would have been 5 weeks pregnant. I got a negative. I took multiple tests during the morning for the next week and got all negatives. However, at 6 weeks, I finally got a positive pregnancy test, and that positive was still somewhat faint.
Because I had a previous miscarriage in my first pregnancy and I had had so many negative tests, at the doctor, after seeing that my pregnancy test showed positive, my doctor ordered that we draw blood to check where my hCG levels were at. Of course, we had to check them after 48 hours as well. When I went back to have my blood drawn the second time, my doctor said that my hCG levels were REALLY low and that didn't indicate a viable pregnancy, but that we would check to make sure that they doubled. I remember him saying "if this is viable, you are really, really early! like 2 and a half weeks pregnant!" I knew that that wasn't right. Anyway, when the second blood draw came back, my hCG levels had, in fact, doubled in 48 hours. The next week, when we did a dating ultrasound (my doctor thought that I would be so early that we wouldn't see anything via ultrasound), we saw a 7 week 3 day old fetus! For whatever reason, I had produced a very low amount of hCG but I'm now 20 weeks and haven't had any complications. Lord willing, we will have our baby with us in April.