Hi!
Congratulations!
Count your blessings! The last thing you need to deal with right now is your period!
If you are breastfeeding, the lack of a period is caused by that--breastfeeding suppresses menstruation for up to 6 months to a year after delivery.
If you are NOT breastfeeding, it could be that you just haven't resumed your cycle yet due to stress. The only thing more stressful than newborn twins would be triplets.
I hate to even mention this, but you should probably consider a pregnancy test if you are not breast feeding, and not using contraception. If you are not using contraception, you may wish to consider starting, too, that could jump start your cycle.
Good luck!
Dr B
PS
I forgot to mention thyroid as a possibility too--if you have any symptoms--fatigue, constipation, dry skin, hair loss, depression, etc you may wish to do TSH.
Dr B