Pregnancy Due Date Calculator
Find out how many weeks pregnant you are, your estimated due date, and your baby's size — instantly, free, and based on the same method your doctor uses.
Wondering when your baby will arrive? Our free pregnancy due date calculator gives you an instant estimate based on your last menstrual period (LMP) or conception date — the same method your doctor or midwife uses to calculate your due date at your first prenatal visit.
An estimated due date isn't a guarantee — it's a planning reference that guides your prenatal care schedule, from when certain screenings are done to how your pregnancy is monitored as you approach term. Most pregnancies fall somewhere in a normal range around this date, not on the exact day itself, which is why understanding how the number is calculated (and how much to trust it) matters more than the single date alone.
This tool doubles as a full pregnancy week calculator: once you enter your dates, you'll also see your baby's size compared to a familiar object for that week, your current trimester, and a day-by-day countdown — all in one place.
Also searched as: baby due date calculator, conception date calculator, pregnancy due date by week, and IVF due date calculator — this tool covers all of these based on the input method you choose.
How This Due Date Calculator Works
This tool uses Naegele's Rule, the standard method used in prenatal care worldwide: it takes the first day of your last menstrual period and adds 280 days (40 weeks) to estimate your due date. If you're using a conception date instead, it adds 266 days, since conception typically occurs about two weeks after the start of your last period.
The calculator also adjusts for your average cycle length. Naegele's Rule assumes a standard 28-day cycle with ovulation on day 14 — if your cycles typically run longer or shorter than that, the estimate shifts to stay accurate to your body's own pattern rather than applying a one-size-fits-all number.
Due Date Calculation Methods Compared
Not every method of estimating a due date carries the same level of precision. Here's how the main approaches compare, so you understand where your calculated due date sits on that spectrum:
| Method | Typical Accuracy | Best For |
|---|---|---|
| LMP (Naegele's Rule) | ± 7 to 14 days | Regular 28-day cycles |
| First-trimester ultrasound | ± 5 to 7 days | Irregular cycles or unknown LMP |
| Known conception date | ± 3 to 5 days | Tracked ovulation or IUI |
| IVF embryo transfer date | Most precise of all | IVF pregnancies |
If your cycles are irregular, or you're not confident of your LMP date, a first-trimester ultrasound measuring your baby's crown-rump length is generally considered the most reliable way to confirm gestational age — more so than a calculation based on LMP alone. Your care provider may adjust your official due date after that scan, and that adjusted date is the one to go by from that point forward.
How Accurate Is My Due Date?
It's natural to treat your due date as a fixed deadline, but the research tells a different story: only about 4-5% of babies are actually born on their exact estimated due date. Most arrive within a two-week window on either side of it, and a full-term pregnancy is considered anywhere from 37 to 42 weeks.
According to the American College of Obstetricians and Gynecologists (ACOG), pregnancies are grouped into early term (37-38 weeks), full term (39-40 weeks), late term (41 weeks), and post-term (42 weeks and beyond) — all considered within the range of normal. Knowing which window you're in matters more for your care plan than the single calculated date does.
What Happens If You Go Past Your Due Date?
Going past your due date is common and, on its own, not usually a cause for alarm — a meaningful share of first-time pregnancies extend into the 41st week. That said, your care provider will likely increase monitoring the closer you get to 41-42 weeks, since post-term pregnancies carry a somewhat higher risk profile and many providers discuss induction options once you cross into that window.
Why Your Due Date Matters For Prenatal Care
Even with its built-in uncertainty, your estimated due date does real work behind the scenes. It's the reference point your provider uses to time key screenings — like first-trimester genetic testing, the anatomy scan around 20 weeks, and glucose screening in the third trimester. It also helps flag if a pregnancy is progressing as a preterm or post-term case, both of which call for closer monitoring.
Frequently Asked Questions
How is my pregnancy due date calculated?
Your due date is estimated by adding 280 days to the first day of your last menstrual period, a method called Naegele's Rule. If you know your exact conception date instead, 266 days is added from that date.
How accurate is a due date calculator?
Due date calculators give a helpful estimate, but only about 4-5% of babies are born on their exact calculated date. Most births happen within two weeks before or after, and a full-term pregnancy can range from 37 to 42 weeks.
How many weeks pregnant am I?
Enter the first day of your last period into the calculator above, and it will instantly show your current week of pregnancy, based on the number of days since that date divided into weeks.
Can I calculate my due date if my periods are irregular?
Yes, but the estimate will be less precise. This calculator adjusts for your average cycle length, though if your cycles vary significantly month to month, a first-trimester ultrasound from your doctor will give a more reliable estimate.
What's the difference between calculating from LMP versus conception date?
Calculating from your last menstrual period is the standard method, since conception date often isn't known precisely. If you do know your exact conception date — for example, from ovulation tracking — that can offer a slightly more direct estimate.
Is a due date calculator accurate for IVF pregnancies?
IVF pregnancies can actually be dated more precisely than natural conceptions, since the embryo transfer date is known exactly. If you conceived via IVF, your fertility clinic will typically provide your most accurate estimated due date directly.
Does this work as a pregnancy calculator for twins?
The date calculation itself works the same way for twin pregnancies, but twins are statistically more likely to arrive earlier than the standard 40-week estimate — often closer to 36-37 weeks. Your care provider will guide you on a more tailored timeline if you're carrying multiples.
When should I see a doctor to confirm my due date?
It's recommended to schedule your first prenatal visit as soon as you know you're pregnant, ideally within the first 8-10 weeks. Your provider can confirm your estimated due date with an ultrasound, which is typically more accurate than a calculation based on your last period alone.
Sources: American College of Obstetricians and Gynecologists (ACOG) clinical guidance on pregnancy dating and term definitions.
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