Due Date Calculator
Enter the first day of your last menstrual period (LMP). The calculator estimates your due date, current pregnancy week, days remaining, and trimester.
Estimated Due Date
January 5, 2027
Current Week
Days Remaining
Trimester
1st Trimester (Weeks 1–12)
How to Calculate Your Pregnancy Due Date
A pregnancy due date — also called the estimated date of delivery (EDD) — is the projected date when a baby is expected to be born. This calculator uses Naegele's rule, the most widely accepted method in obstetrics, to estimate your due date based on the first day of your last menstrual period (LMP). Understanding your due date helps you and your healthcare provider plan prenatal care, schedule important tests, and prepare for delivery.
Naegele's Rule Explained
Naegele's rule was developed by German obstetrician Franz Karl Naegele in the early 19th century. The formula is simple: add 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day menstrual cycle with ovulation occurring on day 14. The formula can also be stated as: subtract three months from the LMP, add seven days, and adjust the year if needed.
For example, if the first day of your last period was January 1, your estimated due date would be October 8 (January 1 + 280 days). While straightforward, this rule has been the gold standard for estimating due dates for over 200 years and remains the starting point for prenatal care worldwide.
Trimester Breakdown
Pregnancy is divided into three trimesters, each with distinct developmental milestones:
First Trimester (Weeks 1–12)
The first trimester begins on the first day of your last menstrual period, even though conception typically occurs around week 2. During this period, all major organs begin to form, the heart starts beating around week 6, and by week 12 the fetus is about 2.5 inches long. Common symptoms include morning sickness, fatigue, and breast tenderness. Most miscarriages occur during this trimester, which is why many parents wait until week 12 to announce their pregnancy.
Second Trimester (Weeks 13–27)
Often called the "golden trimester," many women feel their best during this period as morning sickness typically subsides. The baby grows rapidly — by week 20 you can usually learn the sex via ultrasound. Fetal movement (quickening) is first felt between weeks 16 and 25. The anatomy scan, one of the most important prenatal ultrasounds, is typically performed between weeks 18 and 22.
Third Trimester (Weeks 28–40+)
The baby gains significant weight during the third trimester, growing from about 2.5 pounds at week 28 to an average of 7.5 pounds at birth. The lungs mature, the brain develops rapidly, and the baby typically moves into a head-down position by week 36. Braxton Hicks contractions become more common, and prenatal visits increase to weekly by week 36.
Key Prenatal Milestones
| Week | Milestone |
|---|---|
| 6–8 | First ultrasound confirms pregnancy and heartbeat |
| 10–13 | First-trimester screening (nuchal translucency) |
| 16 | Quad screen blood test |
| 18–22 | Anatomy scan ultrasound |
| 24–28 | Glucose tolerance test for gestational diabetes |
| 28 | Rh antibody test; third trimester begins |
| 36 | Group B strep test; weekly visits begin |
| 37 | Pregnancy considered early term |
| 39–40 | Full term; labor expected |
| 41 | Late term; induction discussed |
| 42 | Post-term; induction typically recommended |
When Do Babies Actually Arrive?
Despite the precision of due date calculations, only about 4–5% of babies are born on their exact due date. According to research, approximately 80% of babies are born between weeks 37 and 41. First-time mothers tend to deliver slightly later than the due date on average, while subsequent pregnancies may deliver a few days earlier.
A study published in the journal Human Reproduction found that the natural length of pregnancy can vary by as much as 37 days among healthy women. Factors that influence delivery timing include the mother's age, whether she has given birth before, genetic factors, and the baby's sex — girls tend to arrive slightly earlier than boys on average.
Accuracy and Limitations
Naegele's rule assumes a regular 28-day menstrual cycle, which does not apply to all women. If your cycles are longer or shorter than 28 days, the due date estimate may be off by several days. Ultrasound dating in the first trimester (before 14 weeks) is considered the most accurate method, with a margin of error of about ±5 days. Later ultrasounds have wider margins of error.
Your healthcare provider may adjust your due date based on ultrasound measurements, especially crown-rump length in the first trimester. Always rely on your provider's clinical judgment for the most accurate estimated due date.
Frequently Asked Questions
How accurate is the due date calculator?
This calculator uses Naegele's rule (LMP + 280 days), which is the same method used by most healthcare providers as a starting point. It is most accurate for women with regular 28-day cycles. Only about 4–5% of babies arrive on the exact due date, so think of it as an estimate within a two-week window.
What if I don't know the exact date of my last period?
If you are unsure of your LMP, your healthcare provider can estimate gestational age using an early ultrasound (ideally before 14 weeks). Crown-rump length measurements in the first trimester are accurate to within about 5 days.
Does the due date change if my cycle is longer than 28 days?
Yes. If your cycle is longer than 28 days, ovulation likely occurred later, and your actual due date may be a few days later than what Naegele's rule predicts. Subtract 28 from your actual cycle length and add those extra days to the calculated due date.
What is considered full term pregnancy?
The American College of Obstetricians and Gynecologists defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37–38 weeks, late term is 41 weeks, and post-term is 42 weeks and beyond.
Can I use this calculator for IVF pregnancies?
For IVF pregnancies, the due date is typically calculated from the embryo transfer date rather than LMP. Your fertility specialist will provide a more precise due date based on the transfer date and embryo age.
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Sources
- American College of Obstetricians and Gynecologists: Methods for Estimating the Due Date
- Jukic, A.M. et al. (2013). Length of human pregnancy and contributors to its natural variation. Human Reproduction, 28(10), 2848–2855.
- National Institutes of Health: What are the factors that put a pregnancy at risk?
- World Health Organization: Pregnancy