Mark Your Calendar: Your Baby's Big Day Is Coming
The moment you see those two pink lines, one of the first questions racing through your mind is simple: *When will my baby be here?* Whether you're trying to plan the nursery, take time off work, or just daydream about meeting your little one, knowing your due date is the first milestone in your pregnancy journey. This calculator takes the guesswork out of it.
What This Calculator Does
Your due date is typically calculated as 40 weeks (280 days) from the first day of your last menstrual period (LMP), a method called Naegele's Rule. If you know your conception date instead, we can work backward from that too. This calculator instantly computes your due date in both scenarios, saving you the mental math and giving you a reliable date to share with your partner, family, and healthcare provider.
How to Use This Calculator
Using this calculator is straightforward. If you know the first day of your last period, enter that date in the LMP field. The calculator will automatically add 280 days to give you your estimated due date.
If you don't remember your exact LMP or prefer to use your conception date, select that option instead. Conception typically occurs around ovulation, which is roughly 14 days after the start of your cycle, but if you know your conception date (from fertility tracking, IVF records, or other methods), you can use it directly. The calculator adds 266 days to your conception date to estimate your due date.
Keep in mind that due dates are *estimates*. Your healthcare provider may adjust your due date based on early ultrasounds, which are more accurate in the first trimester. It's completely normal for babies to arrive a week or two before or after the calculated date, in fact, only about 5% of babies arrive exactly on their due date.
The Formula Behind the Math
Naegele's Rule is the standard formula obstetricians use worldwide:
Due Date = Last Menstrual Period (LMP) + 280 days (40 weeks)
Or equivalently: LMP + 9 months + 7 days
Here's how it works: A typical pregnancy lasts 40 weeks from the first day of your last period. That's roughly 9 months on the calendar, plus about a week. The reason we count from LMP (not conception) is because LMP is usually easier to remember than the exact day you conceived.
Working Example:
If using conception date:
(The 266-day method accounts for the roughly 14 days between LMP and conception.)
Why the variation? Cycle lengths differ, if your cycle is longer than the average 28 days, you ovulate later, and your actual due date shifts accordingly. Early ultrasounds (especially around 12 weeks) are far more accurate than calculations because they measure your baby's actual development. Our calculator does all of this instantly, but now you understand exactly what it's computing.
Planning Your Maternity Leave
Your due date is essential for workplace planning. Most parents aim to start leave 2–4 weeks before the due date, giving you time to rest and prepare. If you work in a job with physical demands or long commutes, you might go earlier. If you're self-employed or have flexibility, you might work closer to your due date. Knowing your due date lets you negotiate this timeline with your employer well in advance.
Adjusting Your Timeline After Your First Ultrasound
During your first ultrasound (usually around 8–12 weeks), your sonographer measures your baby's length and compares it to expected growth. If your baby is larger or smaller than expected for your LMP date, your due date may be adjusted. Don't panic, this is normal and common. Ultrasound measurements are more accurate in early pregnancy, so trust your provider's revised date. By the third trimester, ultrasound dating becomes less reliable, so early adjustments matter most.
Managing Expectations as Your Due Date Approaches
As your due date draws near, it's easy to fixate on that calendar date, checking it daily, wondering if today is the day. The reality is that only about 5% of babies arrive exactly on their due date. Most arrive within a week or two before or after. Your healthcare provider will typically let you go 2 weeks past your due date before discussing induction (unless there are other medical factors). This "due date window" is actually about 4 weeks long, which takes some of the pressure off.
Tips and Things to Watch Out For
Cycle length matters. This calculator assumes an average 28-day cycle. If your cycle is typically longer (say, 32 days) or shorter (24 days), your actual ovulation date, and thus your due date, may shift. Talk to your healthcare provider about adjusting your date if you have irregular cycles.
Dating scans are the gold standard. An ultrasound in your first trimester is far more accurate than any calculation. If your ultrasound shows a different due date, trust it over the calculator. Your baby's development doesn't lie.
Conception date confusion. If you used fertility apps or tracking, your "conception date" may be an estimate, not a fact. Sperm can survive for several days, and you might ovulate a day or two earlier or later than predicted. The range of possible conception dates is typically about 3 days. When in doubt, use your LMP date instead.
Multiple pregnancies change the timeline. If you're carrying twins, triplets, or more, your due date is often earlier. Your healthcare provider will adjust this for you during early ultrasounds.
Posterior cervix, shorter cycle, or unusual bleeding? Some people have spotting or bleeding during early pregnancy that they mistake for their period. If you're unsure about your LMP, ask your healthcare provider to date your pregnancy via ultrasound.
This calculator provides general information only. Always consult your OB-GYN, midwife, or pediatrician for medical guidance specific to your pregnancy and baby.
Frequently Asked Questions
When should I input my due date?
Enter the first day of your last menstrual period or your conception date. Your healthcare provider may adjust this date after your first ultrasound, so don't worry if it shifts by a week or so, that's completely normal.
Can I be overdue if I pass my due date?
Yes, but "overdue" doesn't mean unsafe right away. Babies are considered full-term at 39 weeks. Most providers allow pregnancy to continue until 41–42 weeks before recommending induction, especially if you and your baby are healthy.
What's the difference between my calculated due date and my ultrasound due date?
Ultrasound measurements in the first trimester are more accurate than date calculations. If your ultrasound due date differs from your calculated date, trust the ultrasound. Your provider will update your records accordingly.
If I conceived via IVF, should I use a different date?
Yes, use our IVF Due Date Calculator instead. IVF dates are calculated from your transfer date, not your LMP, because the exact timing of fertilization and embryo development is known.
Do all pregnancies last exactly 40 weeks?
Most full-term pregnancies last between 37 and 42 weeks. Your baby will arrive when ready, and your due date is simply the midpoint, a helpful marker, not a firm deadline.
Can my due date change during pregnancy?
It can be adjusted after your first ultrasound if your baby's development suggests a different date. Later ultrasounds are less precise, so adjustments are less common after the first trimester.
Related Calculators
Knowing your due date is just the beginning. Use our Pregnancy Week Calculator to see exactly how many weeks along you are right now, and which trimester you're in. If you're curious about when conception likely occurred or want to explore your ovulation window, try our Conception Date Calculator or Ovulation Calculator. And if you conceived via IVF, our IVF Due Date Calculator will give you a more precise estimate based on your transfer date.