How this due date calculator works
Most pregnancy calendars estimate a due date by counting about 280 days (40 weeks) from the first day of your last menstrual period (LMP). If your average cycle is longer or shorter than 28 days, the calculator shifts that estimate so ovulation timing is closer to your pattern.
That means “week 1” on a pregnancy calendar is often the week of your period — before conception. It sounds odd at first, but it keeps clinicians speaking the same dating language.
LMP dating vs ultrasound dating
LMP math is a starting point. An early ultrasound can refine gestational age based on measurements. If your clinician gives you a different due date, trust that clinical date in Preglo. This free tool never replaces medical dating, especially with irregular cycles, fertility treatment, or uncertain period dates.
What your result is useful for
- Finding your approximate current week in the week-by-week guide
- Planning prenatal appointment windows with your care team
- Setting a calm baseline in Preglo so symptoms and kicks sit on the right week
After you calculate
Open your week page, skim what typically changes, then download Preglo to log the parts only you can feel — energy, nausea, water, movements, and notes you want private. Later in pregnancy, pair dating with kick counting guidance.
Estimates only — not medical advice. Confirm dates with your care provider before making health decisions.