Pregnancy planning
Reading the pregnancy due date calculator
Use your last menstrual period or an early ultrasound to estimate the delivery date and map prenatal visits.
Free online tool
Due date calculator
Where the due date comes from
The estimated due date (EDD) is calculated using Naegele's rule, a formula published by German obstetrician Franz Karl Naegele in the early 19th century. The calculation is simple: take the first day of the last menstrual period (LMP), add 280 days (40 weeks), and that is your due date. This rule assumes a 28-day cycle with ovulation on day 14, which means it works best for women with regular cycles.
Despite its age, Naegele's rule remains the starting point for obstetric dating worldwide. According to the American College of Obstetricians and Gynecologists (ACOG), the EDD should be confirmed or adjusted by first-trimester ultrasound whenever possible, since only about 5% of babies are actually born on their estimated due date.
Methods for calculating the due date
Last menstrual period (LMP)
This is the most common starting point. You enter the first day of your most recent period, and the calculator adds 280 days. The method is straightforward but has known limitations: women with irregular cycles, those who do not remember the exact date, and those who were using hormonal contraception shortly before conception may get inaccurate results.
First-trimester ultrasound
An ultrasound performed between 8 and 13 weeks of pregnancy measures the crown-rump length (CRL) of the embryo and estimates gestational age with an accuracy of plus or minus 5-7 days. If the ultrasound-based date differs from the LMP-based date by more than 5 days, ACOG recommends using the ultrasound date. First-trimester ultrasound is considered the gold standard for pregnancy dating.
Known conception date
If you know the exact date of conception (for example, through timed intercourse or intrauterine insemination), add 266 days (38 weeks) to get the EDD. This is equivalent to the Naegele calculation minus the 14-day difference between the LMP and ovulation.
IVF transfer date
For pregnancies achieved through IVF, the due date is calculated from the embryo transfer date. For a day-5 blastocyst transfer, add 261 days (or equivalently, set the LMP to 19 days before the transfer date). This method is the most precise because the age of the embryo is known exactly.
Why only 5% of babies arrive on the due date
The term 'due date' suggests precision, but human biology is inherently variable. A 2013 study published in Human Reproduction that tracked natural conceptions found that the median duration of pregnancy is about 268 days from ovulation, with considerable individual variation. Factors that influence when labor begins include:
- First pregnancies tend to last slightly longer than subsequent ones.
- Maternal age: Women over 35 have a slightly higher rate of post-term pregnancy.
- Genetics: If your mother or sister went past their due date, you may be more likely to as well.
- Fetal sex: Some studies suggest that male fetuses are carried slightly longer on average.
- Ethnicity: The standard 280-day calculation was derived primarily from European populations and may not be equally accurate for all groups.
For this reason, most obstetricians frame the due date as the center of a 'due window' spanning from 37 to 42 weeks. The ACOG classification system defines early term (37-38 weeks), full term (39-40 weeks), late term (41 weeks), and post-term (42+ weeks).
Trimester roadmap: tests and milestones
Trimester checklist
First trimester (weeks 1-12)
Foundation phase
Confirm pregnancy, establish the due date via ultrasound, begin prenatal vitamins (especially folic acid 400-800 mcg daily), complete first-trimester screening for chromosomal conditions, baseline blood work (blood type, Rh factor, CBC, rubella immunity, STI screening), and discuss medication safety.
Second trimester (weeks 13-27)
Monitoring phase
Anatomy scan at 18-22 weeks to check fetal organs and structure. Glucose screening for gestational diabetes between 24-28 weeks. Rh immunoglobulin injection at 28 weeks if the mother is Rh-negative. Begin childbirth education classes. Track fetal movements starting around 20 weeks.
Third trimester (weeks 28-40)
Preparation phase
Growth scans if indicated. Group B Streptococcus (GBS) screening at 36-37 weeks. Finalize birth plan. Weekly or biweekly prenatal visits after 36 weeks. Discuss induction criteria if pregnancy extends past 41 weeks. Prepare hospital bag and support network.
Using the due date calculator
- Enter the first day of your last menstrual period (LMP). If you know the conception date or have an early ultrasound measurement, use it as an alternative input.
- The calculator will display the estimated delivery date, the current gestational week, and the trimester you are in.
- Note the key milestones for your current week and plan upcoming appointments accordingly.
- Share the timeline with your partner, family, and prenatal care team so everyone is aligned.
- Revisit the calculation if your provider adjusts the due date based on ultrasound findings.
Important note for irregular cycles
If your menstrual cycles are longer than 35 days, shorter than 21 days, or highly variable, the LMP-based calculation may be off by a week or more. An early ultrasound is strongly recommended to establish an accurate due date.
When to recalculate the due date
According to ACOG, the due date should be set as early as possible in pregnancy and ideally confirmed by first-trimester ultrasound. Once established, it should not be changed based on later ultrasounds because fetal size variability increases as pregnancy progresses. However, there are specific situations where recalculation is appropriate:
- The first-trimester ultrasound differs from the LMP-based date by more than 5 days.
- No reliable LMP is available and the first ultrasound occurs in the second trimester (accuracy drops to plus or minus 7-10 days).
- Assisted reproduction where the exact embryo age is known and conflicts with LMP-based dating.
What to do as the due date approaches
As you enter the final weeks of pregnancy, focus on practical preparation:
- Pack a hospital bag by 36 weeks with essentials for you and the baby.
- Finalize your birth plan, including preferences for pain management, delivery position, and skin-to-skin contact.
- Discuss the signs of labor with your provider: regular contractions, rupture of membranes, and bloody show.
- Know when to go to the hospital: contractions every 5 minutes lasting 1 minute each for at least 1 hour (the 5-1-1 rule), water breaking, or decreased fetal movement.
- Arrange post-delivery support: meals, help with older children, and breastfeeding resources.
Frequently asked questions
Can I go past my due date?
Yes. About 10% of pregnancies continue beyond 41 weeks. Most providers will discuss induction between 41 and 42 weeks because the risk of stillbirth and placental insufficiency increases after this point. The ARRIVE trial (2018) showed that elective induction at 39 weeks in low-risk first pregnancies did not increase cesarean rates and reduced the risk of hypertensive disorders.
Is a second-trimester ultrasound accurate for dating?
It is less accurate than a first-trimester ultrasound. A second-trimester scan has a margin of error of plus or minus 7-10 days, compared to plus or minus 5 days in the first trimester. If no earlier dating is available, it is still better than LMP alone.
This article is for educational purposes only and does not replace professional medical advice. Always consult your obstetrician or midwife for personalized prenatal care.
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