Pregnancy Due Date Calculator: When Will Your Baby Actually Arrive?

The pregnancy due date calculator finds your estimated delivery date from your last menstrual period — for the conception dates and fertile window behind this pregnancy, use the Ovulation Calculator.

Why Your Due Date Matters More Than Most People Realize

Most pregnant women know roughly when their baby is coming — but without a specific calculated date, planning maternity leave, childcare, and prenatal appointments becomes guesswork that costs real money.

According to the American College of Obstetricians and Gynecologists, only 4% of babies are born on their exact due date, but approximately 80% arrive within 2 weeks of it. That 4-week window centered on your due date is the most critical planning period of early parenthood — and it starts with knowing the right date.

People who never calculate a proper estimated due date often miss childcare placement windows — many nurseries require booking 6 to 12 months in advance. Missing an early registration deadline can cost $1,200 to $3,000 in premium last-minute placements or extended unpaid leave while waiting for a spot.

Pregnancy Due Date Explained in Plain English

A pregnancy due date is the estimated date your baby is expected to arrive, calculated from the first day of your last menstrual period — even though conception actually happens about 2 weeks later. The reason doctors count from your last period rather than conception is that most women know that date accurately, while the exact conception date is harder to pinpoint.

This due date calculator uses a standard that has been applied in obstetrics for over 200 years. It treats every pregnancy as lasting 280 days — 40 weeks — from that last period date. Your result is called your estimated due date, or EDD — an approximation based on average gestational length, not a guarantee of the birth day.

How to Calculate Your Due Date — Step by Step

Naegele’s Rule: EDD = Last Menstrual Period + 280 days

Last Menstrual Period (LMP) is the first day of your most recent period before this pregnancy — the date your doctor always asks for at the first prenatal appointment. If your last period started on January 5, that is your LMP. Every other calculation in pregnancy dating builds from this single date, so accuracy here matters — being off by even one week shifts every trimester boundary and milestone that follows.

Gestational Length is the standard 280 days — 40 weeks — that Naegele’s Rule adds to your LMP. This number comes from population averages of pregnancy duration and includes the approximately 2 weeks between your period and actual ovulation, plus the 38 weeks of actual fetal development. A 28-day menstrual cycle is the assumed baseline; cycles longer or shorter may shift the due date slightly.

Adjusted Due Date is the result — your LMP plus 280 days produces the specific calendar date of your EDD. Adding 280 days to January 5 produces October 12 as the due date. Once you have your EDD, use the Pregnancy Trimester Calculator to find the exact start and end dates of each trimester and the key milestones within them.

Worked Example: Amara Calculates Her Estimated Due Date

Amara is 29 years old and her last menstrual period started on March 14. She has just received a positive pregnancy test and wants to know when to expect her baby before booking her first midwife appointment.

She applies Naegele’s Rule: March 14 + 280 days. March has 31 days, leaving 17 remaining in March. 280 − 17 = 263 remaining days from April 1 onward. Counting forward: April (30) + May (31) + June (30) + July (31) + August (31) + September (30) + October (31) + November (30) = 244 days. 263 − 244 = 19 days into December. Due date: December 19.

Amara’s estimated due date is December 19 — placing her in a winter delivery window with 39 weeks to prepare.

Armed with her date, Amara books her first prenatal appointment for week 8 — approximately May 9 — and immediately registers at her preferred childcare center, which has a 9-month waiting list. She also begins reviewing her employer’s maternity leave policy to confirm she qualifies for the maximum 39 weeks of statutory leave.

What to Do with Your Pregnancy Due Date Result

• Run the pregnancy due date calculator above as soon as you have a positive test — every week of delay is a week lost for appointment booking, childcare registration, and employer notification planning.
• Book your first prenatal appointment for week 8 to 10 — this is when your first ultrasound is typically scheduled, and an early scan will confirm or adjust your due date by measuring fetal size directly.
• Use the Pregnancy Weight Gain Calculator immediately after getting your due date — your pre-pregnancy BMI and due date together determine the specific weight gain range recommended for your entire pregnancy.
• Do not treat your due date as a delivery deadline. Counter-intuitively, planning as if your baby could arrive 2 weeks early — at 38 weeks — protects you far better than preparing for the exact date. Hospitals consider births between 37 and 42 weeks to be full term.

Pregnancy Due Date: 5 Common Questions Answered

Q: What is an estimated due date and how accurate is it? A: Your estimated due date is the calculated 40-week mark from your last period. Only 4% of babies arrive on this exact date. The more useful planning window is the 4-week range from 38 to 42 weeks — 95% of full-term deliveries fall within this span.

Q: Is pregnancy really 9 months long? A: No — and this is the most common misconception about pregnancy length. Pregnancy is 40 weeks, which equals 10 lunar months of 28 days each, or closer to 9.2 calendar months. Counting from your last period to your due date always produces a number closer to 9.5 months than 9 months on the calendar.

Q: Can my due date change after my first ultrasound? A: Yes — if an early ultrasound shows fetal measurements that differ significantly from the LMP-based date, your doctor may revise the EDD. A difference of more than 7 days in the first trimester or 14 days in the second trimester typically results in a new official due date.

Q: What if I do not know my last menstrual period date? A: Your doctor will use an early ultrasound — ideally before 13 weeks — to estimate gestational age from fetal crown-to-rump length. This measurement is accurate to within 5 to 7 days in the first trimester and remains the most reliable dating method when the LMP is unknown.

Q: Does my cycle length affect my due date? A: Yes. Naegele’s Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is consistently 35 days, ovulation likely occurs around day 21 — approximately 7 days later than the formula assumes. A longer cycle generally shifts the due date approximately 7 days later than the standard calculation produces.

Related

Related: Ovulation Calculator | Pregnancy Trimester Calculator