Start from the first day of your last menstrual period
Count the number of days from that date to today
Divide the total days by 7 to get the number of weeks
The remaining days are the extra days in your pregnancy week
Example: 35 days since your last period equals 5 weeks and 0 days
Use your estimated due date if you know it
Subtract 280 days from the due date to estimate the first day of your last period
Ask your doctor or midwife for an ultrasound-based pregnancy dating if your cycle is irregular
Use a pregnancy calculator or app for a quick estimate