Re: at what week will they be able to detect twins?
An ultrasound is really the only way twins can be confirmed. I'm expecting triplets and my Dr was able to confirm this at 5 weeks 3 days along when three gestational sacs and fetal poles were noted. It is possible that a doctor might miss a multiple pregnancy in an ultrasound that early unless they are specifically looking for it (as was my Dr). As for signs, well there really aren't any reliable signs. Some may say intense morning sickness (I had NO m/s), rapid weight gain and growth (which takes some time to develop), or extrememly high HCG levels early on (which was the case with me, but I've also seen that NOT to be the case with multiples). The only problem with these "signs" is that they can also be present in a singleton pregnancy. If you suspect you are expecting twins, I'd recommed getting in for an u/s to confirm. Good luck!