It's very difficult calculate if you have irregular periods.
I take it your periods was regular before you got pregnant? Fist you need to find out the reason why your period has become irregular then you can work on putting it back to the way it was.
Stress is a common cause of irregular periods. If you are fatigued, worried, or anxious this can cause your hormones to become unbalanced.
How long have you been having irregular periods?
If you have only been experiencing irregular periods for a short time (less than 7 months), it is likely that your periods will become regular again on their own.