Is there a cutoff for that 12 month period to be considered common law? because we moved in together last April which would make it 12 months under the fiscal year

In order to be considered common law, you need to have been living together in a conjugal relationship for 12 months as of December 31, 2017, to claim common law on your income tax return.  However, if you have a child together, you are considered common law even if you have not been living together for 12 months.