Pregnancy Due Date Calculator
The wait is 40 weeks. Elephants wait 22 months. Perspective achieved.
About This Calculator
How It Works
This calculator uses Naegele's Rule — the standard method used by OBs and midwives worldwide — to estimate a due date. From the first day of your last menstrual period (LMP), it adds 280 days (40 weeks). If you know your conception date instead, it adds 266 days. It also calculates how many weeks pregnant you currently are and which trimester you're in, giving you a fuller picture of where you are in the pregnancy timeline.
The Formula
Naegele's Rule: LMP + 280 days (40 weeks) = Estimated Due Date. From conception: Conception Date + 266 days.
When to Use This
Use this right after you get a positive test to get your bearings on a timeline before your first prenatal appointment. It's also useful for planning parental leave, coordinating with family, or just satisfying the immediate need to know 'when.' The LMP method is the standard starting point; your doctor will confirm or adjust the date based on your first ultrasound, which is more accurate in early pregnancy.
Limitations
Naegele's Rule assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter, or irregular, the calculated due date may be off by a week or more. Only about 5% of babies actually arrive on their estimated due date — full-term ranges from 37 to 42 weeks. This calculator gives you an educated target, not a schedule. Always confirm dates with your healthcare provider.
Example
Last period: January 1 → Due date: October 8. (About 9 months and 7 days. Give or take — only 5% of babies arrive on their exact due date.)
Did You Know?
Elephants are pregnant for 22 months — nearly two years. Meanwhile, the shortest mammal pregnancy belongs to the Virginia opossum at just 12 days. Humans, at 40 weeks, are somewhere reasonably in the middle.
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One of the First Things Your Doctor Will Recommend
You've got your due date — congratulations. Now comes the part where everyone starts telling you what to do, what to eat, what to avoid, and what to take. It's a lot. But there's one thing that's genuinely non-negotiable from day one: prenatal vitamins.
Prenatal vitamins aren't optional extras — they're the foundation of fetal development during the first trimester, when your baby's neural tube, heart, and major organs are forming. Folic acid (critical for preventing neural tube defects), DHA (essential for brain and eye development), and iron (to support increased blood volume) are the three most important nutrients, and getting enough of each through diet alone during pregnancy is extremely difficult.
Starting prenatal vitamins as early as possible — ideally before you even get a positive test — is one of the most impactful things you can do for your baby's development.
Our pick: Nature Made Prenatal Vitamins with Folic Acid + DHA
Nature Made Prenatal is one of the most widely recommended prenatal vitamins by OB-GYNs and pharmacists, and one of the top-selling prenatal supplements on Amazon. It's third-party tested by USP (U.S. Pharmacopeia), which independently verifies that what's on the label is actually in the capsule — an important distinction in a supplement category that isn't heavily regulated. Each softgel provides folic acid, DHA, iron, and 14 additional vitamins and minerals in a single daily dose. A 110-day supply means you're covered for the entire first trimester and well into the second. Under $20 and available for same-day delivery in most areas.
If you've just confirmed your pregnancy dates and you haven't started prenatal vitamins yet — this is the one to start with.
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