Hi geethaprasad
i understand from the pregnancy test I have read today, that they should be able to give you an answer from the first day your period is due. I have been reading many information sites and there is a possibility that sometimes the hormone levels are not high enough for the pregnancy tests to pick up, so the best option is to go and see your gp for a proper blood test.
I have also found out that you should speak with your gp when you are planning on getting pregnant, they can then offer advice and support.
Good luck and I hope that you are pregnant....That would be lovely!
Xx