Pregnancy Due Date Calculator — Week by Week Guide for Expecting Mothers

Pregnancy Due Date Calculation — How Estimated Dates Are Determined

The moment a pregnancy is confirmed, the first question is almost always “when is the baby due?” The answer is based on a calculation method that has been used for over a century, is surprisingly simple in its math, and comes with an important caveat that many parents do not fully appreciate: a due date is an estimate, not an appointment.

How Due Dates Are Calculated

The standard method, known as Naegele’s rule (named after the German obstetrician who formalized it in the early 1800s), calculates the estimated due date as: First day of last menstrual period (LMP) + 280 days (40 weeks).

This calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14. Since conception typically occurs around ovulation, the 280-day calculation actually estimates 266 days from conception plus 14 days from the start of the last period to ovulation.

For example: if the first day of your last period was January 15, your estimated due date is October 22 (January 15 + 280 days).

A Shortcut That Doctors Use

A quick mental math version: take the first day of your LMP, subtract 3 months, and add 7 days. LMP of January 15: subtract 3 months = October 15, add 7 days = October 22. This shortcut produces the same result as counting 280 days and is how many obstetricians calculate due dates in their head during initial consultations.

When the Standard Calculation Does Not Apply

The 280-day calculation assumes a textbook 28-day cycle, but menstrual cycles vary significantly among individuals. If your cycle is regularly 35 days, you likely ovulate around day 21 instead of day 14, meaning you should add 7 days to the standard due date. If your cycle is 25 days, ovulation may occur around day 11, and you should subtract 3 days.

For individuals with irregular cycles, the LMP-based calculation can be significantly off. In these cases, dating ultrasounds performed during the first trimester (typically between weeks 8 and 13) provide more accurate due date estimates by measuring the embryo’s crown-to-rump length, which correlates closely with gestational age during early pregnancy.

How Accurate Are Due Dates?

Here is the statistic that surprises most expecting parents: only about 4-5% of babies are born on their estimated due date. The vast majority of full-term deliveries occur within a two-week window around the due date — anywhere from 38 to 42 weeks of gestation is considered normal.

First pregnancies tend to go slightly past the due date on average. The median delivery date for first-time mothers is about 5 days after the EDD. Subsequent pregnancies tend to be closer to or slightly before the estimated date.

Week-by-Week Pregnancy Milestones

Understanding where you are in the 40-week journey helps contextualize prenatal care schedules and fetal development:

  • Weeks 1-4: Implantation occurs. Most women do not yet know they are pregnant.
  • Weeks 5-8: First missed period. Heartbeat detectable by ultrasound around week 6. Morning sickness often begins.
  • Weeks 9-12: First trimester screening. Risk of miscarriage decreases significantly after week 12.
  • Weeks 13-16: Second trimester begins. Energy often returns. Baby begins moving (though usually not felt until weeks 16-20).
  • Weeks 17-20: Anatomy scan ultrasound (typically around week 20). Baby’s sex can often be determined. First fetal movements felt.
  • Weeks 21-28: Rapid growth phase. Viability outside the womb increases significantly after week 24.
  • Weeks 29-36: Third trimester. Baby gains weight rapidly. Prenatal visits increase in frequency.
  • Weeks 37-40: Full term. Baby is fully developed and could arrive any day.

What the Due Date Means Medically

Your due date is the clinical reference point around which all prenatal care is scheduled — blood tests, ultrasounds, screenings, and check-up frequency are all timed relative to your EDD. It also determines when medical intervention may be considered: pregnancies extending beyond 41-42 weeks are monitored more closely and may require induction due to increased risks associated with post-term pregnancy.

Use our Pregnancy Due Date Calculator to estimate your due date based on your last menstrual period or conception date, and see week-by-week milestones for your pregnancy journey.