I went for an u/s 6 weeks after last menstrual period(last week). There was nothing. I went back a week later (today) so should be about 7 weeks but still nothing. Last period was on the 5 June 2013. The nurse told me to come back again next week. I am 100% sure of my lmp date. And my period is very regular (every 28-29 days). I read somewhere that conception can happen up until 20 days after day 1 of lmp. Is this true? If it is then it would explain a lot. Otherwise please advice. Thanku
Ps- how many weeks generally until scan shows fetus?

The embryo should be very visible at 7 weeks. Did you test positive before having 2 scans? Maybe you are just skipping a month. 'Very regular' is always going to be subject to that month or year when you change or aren't regular for a while.
I'm sure there are women who have conceived after the 20th day. There are many variables regarding ovulation, plus the sperm can take 3 days to meet and greet the egg.