Is It Too Early To Detect Twins?

I had an ultrasound at 5 weeks and only 45 days from Conception date. Doctor could not see anything yet. Is it because it's just way to early to see anything. I go back in 4 weeks and he plans on doing an ultrasound then. He did say I was very big for only being 5 weeks. He said sometimes it just takes longer to detect and for implantation to occur. I was just wondering if this has occured to anyone els. I have tender breast, fatigue, morning sickness, round ligament pain etc.