is glucophage is safe to take for pregnant diabetic women?

Yes and for women with type 2 diabetes or gestational diabetes it is now, in the UK, considered as standard therapy. There has been a recent trial using metformin in pregnancy. http://content.nejm.org/cgi/content/short/358/19/2003

Basically those on metformin did no better or worse than those women that injected insulin but women preferred to take it instead of insulin. About half of women who took metformin ended up injecting insulin. Metformin is nto formally liscenced for use in pregnancy.

what are the advantages and disadvantages of this medicine?

As above. Metformin has side effects - mainly stomach pains, flatulence, diarrhoea. If you get side effects it should be stopped and insulin prescribed instead.

what are the best food for diabetic pregnant women?

Your dietician should be able to give you advice on this.

 is it normal to have a blood spot for a diabetic pregnant women for almost 6 weeks pregnant?

No. Women can suffer light bleeding in early pregnancy but it is not diabetes related. You should seek medical attention.