Q&A: I am not sure how to tell when I got pregnant.
I don't know whether to date my pregnancy based on my last period, which was short, or my last regular period. Which should I use?
Officially, you should base your due date on your last regular menstrual period, but exceptions abound using this rule of thumb. And for some women, as you’ve discovered, it’s not easy to remember or determine when your last period began.
If you’re at all unsure about whether your lighter period was really your period, don’t worry there’s an easy way to accurately figure out when your baby’s due—an ultrasound. Your health care provider will request you have an ultrasound somewhere around your 20 week time (give or take a few weeks).
Based on measurements of your developing baby taken during the ultrasound, your health care provider should be able to give you a fairly accurate due date.