How will this work🤔

So I’m getting my Nexplanon out tomorrow and will ttc #3. My question is since there is no possible way I’m pregnant now and I’ve checked cause my periods can sometimes be irregular. I had my period on Christmas lasted about 5-6days(normal period) if I were to get pregnant right away and miss my next period will they start my pregnancy as of the 13th instead of the first day of my period 🤔