Due Date Calculator
Use this due date calculator to estimate your pregnancy due date from your last period, conception date, or ultrasound.
Calculated instantly in your browser — nothing is sent to or stored on our servers.
Kimberly Langdon, MD
Kimberly Langdon, MD, is a retired, board-certified obstetrician/gynecologist with 19 years of clinical practice, trained at The Ohio State University.
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Due Date Is Calculated
Your due date is an estimate of when you'll reach 40 weeks of pregnancy, calculated from whichever dating information you have available. ACOG recommends using the earliest, most reliable data on hand, since dating accuracy generally improves the earlier in pregnancy it's established. Here's exactly how each method works.
What This Calculator Measures
This calculator estimates your due date from one of three starting points: your last menstrual period, a known conception date, or an early ultrasound. It does not predict your actual delivery date, which naturally varies by several weeks even in healthy pregnancies.
The Formula
From LMP: Due date = LMP + 280 days
From conception date: Due date = Conception date + 266 days
From ultrasound: Due date = Ultrasound date + (280 minus gestational age in days at the scan)
All three formulas trace back to Naegele's Rule, a 19th-century method still used in clinical dating today¹. The conception and ultrasound formulas are direct mathematical extensions of the same 280-day estimate.
How This Calculator Computes Your Result
Pick whichever starting point you have: LMP with your average cycle length, a known conception date, or an ultrasound date with gestational age at the scan. If your cycle isn't exactly 28 days, we adjust the LMP-based estimate to account for the difference. Each path runs its own formula above and returns a single due date.
What Your Result Means
Term Window | Weeks |
Early Term | 37 weeks to 38 weeks, 6 days |
Full Term | 39 weeks to 40 weeks, 6 days |
Late Term | 41 weeks to 41 weeks, 6 days |
Post-Term | 42 weeks and beyond |
Your due date is the center of this range, not a deadline. ACOG defines these term windows because babies born at different points within them have measurably different average outcomes¹.
Cross-Checking When Multiple Dates Are Available
If you enter both an LMP and an ultrasound date and they disagree, we flag it rather than silently picking one. Per ACOG guidance, an early ultrasound should generally take precedence over LMP-based dating when the two disagree meaningfully, since first-trimester ultrasound is the most accurate dating method available¹, a finding also supported by direct comparisons of ultrasound and menstrual dating³.
A Note on Accuracy
Only about 4 to 5% of babies are born exactly on their estimated due date², and pregnancy length naturally varies by several weeks even without complications. LMP-based dating assumes a 28-day cycle with ovulation on day 14, which doesn't hold for everyone. Treat your due date as the center of a range, not a deadline.
Frequently Asked Questions
How is my due date calculated?
From your last period, due date equals LMP plus 280 days, adjusted for cycle length¹. From a known conception date, we add 266 days, and from an ultrasound, we use the scan date plus the remaining days to reach 280 total.
Is ultrasound dating more accurate than my last period?
Yes, especially in the first trimester. Ultrasound measurements of the embryo are generally more accurate than LMP-based dating alone, particularly for anyone with irregular cycles¹, a finding also borne out by direct comparisons of the two methods³.
Will I actually deliver on my due date?
Probably not exactly. Only about 4 to 5% of babies arrive on their estimated due date, though most arrive within about two weeks of it either way².
What do early term, full term, late term, and post-term mean?
ACOG's official windows relative to your due date¹: early term is 37 to 38 weeks, full term is 39 to 40 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond.
Sources
Methods for Estimating the Due Date, ACOG Committee Opinion No. 700, 2017
Length of Human Pregnancy and Contributors to Its Natural Variation, Jukic et al., 2013
Estimating the Date of Confinement: Ultrasonographic Biometry Versus Certain Menstrual Dates, Mongelli et al., 1996
Due Date Calculator
Use this due date calculator to estimate your pregnancy due date from your last period, conception date, or ultrasound.
Calculated instantly in your browser — nothing is sent to or stored on our servers.
Kimberly Langdon, MD
Kimberly Langdon, MD, is a retired, board-certified obstetrician/gynecologist with 19 years of clinical practice, trained at The Ohio State University.
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Due Date Is Calculated
Your due date is an estimate of when you'll reach 40 weeks of pregnancy, calculated from whichever dating information you have available. ACOG recommends using the earliest, most reliable data on hand, since dating accuracy generally improves the earlier in pregnancy it's established. Here's exactly how each method works.
What This Calculator Measures
This calculator estimates your due date from one of three starting points: your last menstrual period, a known conception date, or an early ultrasound. It does not predict your actual delivery date, which naturally varies by several weeks even in healthy pregnancies.
The Formula
From LMP: Due date = LMP + 280 days
From conception date: Due date = Conception date + 266 days
From ultrasound: Due date = Ultrasound date + (280 minus gestational age in days at the scan)
All three formulas trace back to Naegele's Rule, a 19th-century method still used in clinical dating today¹. The conception and ultrasound formulas are direct mathematical extensions of the same 280-day estimate.
How This Calculator Computes Your Result
Pick whichever starting point you have: LMP with your average cycle length, a known conception date, or an ultrasound date with gestational age at the scan. If your cycle isn't exactly 28 days, we adjust the LMP-based estimate to account for the difference. Each path runs its own formula above and returns a single due date.
What Your Result Means
Term Window | Weeks |
Early Term | 37 weeks to 38 weeks, 6 days |
Full Term | 39 weeks to 40 weeks, 6 days |
Late Term | 41 weeks to 41 weeks, 6 days |
Post-Term | 42 weeks and beyond |
Your due date is the center of this range, not a deadline. ACOG defines these term windows because babies born at different points within them have measurably different average outcomes¹.
Cross-Checking When Multiple Dates Are Available
If you enter both an LMP and an ultrasound date and they disagree, we flag it rather than silently picking one. Per ACOG guidance, an early ultrasound should generally take precedence over LMP-based dating when the two disagree meaningfully, since first-trimester ultrasound is the most accurate dating method available¹, a finding also supported by direct comparisons of ultrasound and menstrual dating³.
A Note on Accuracy
Only about 4 to 5% of babies are born exactly on their estimated due date², and pregnancy length naturally varies by several weeks even without complications. LMP-based dating assumes a 28-day cycle with ovulation on day 14, which doesn't hold for everyone. Treat your due date as the center of a range, not a deadline.
Frequently Asked Questions
How is my due date calculated?
From your last period, due date equals LMP plus 280 days, adjusted for cycle length¹. From a known conception date, we add 266 days, and from an ultrasound, we use the scan date plus the remaining days to reach 280 total.
Is ultrasound dating more accurate than my last period?
Yes, especially in the first trimester. Ultrasound measurements of the embryo are generally more accurate than LMP-based dating alone, particularly for anyone with irregular cycles¹, a finding also borne out by direct comparisons of the two methods³.
Will I actually deliver on my due date?
Probably not exactly. Only about 4 to 5% of babies arrive on their estimated due date, though most arrive within about two weeks of it either way².
What do early term, full term, late term, and post-term mean?
ACOG's official windows relative to your due date¹: early term is 37 to 38 weeks, full term is 39 to 40 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond.
Sources
Methods for Estimating the Due Date, ACOG Committee Opinion No. 700, 2017
Length of Human Pregnancy and Contributors to Its Natural Variation, Jukic et al., 2013
Estimating the Date of Confinement: Ultrasonographic Biometry Versus Certain Menstrual Dates, Mongelli et al., 1996
Due Date Calculator
Use this due date calculator to estimate your pregnancy due date from your last period, conception date, or ultrasound.
Calculated instantly in your browser — nothing is sent to or stored on our servers.
Kimberly Langdon, MD
Kimberly Langdon, MD, is a retired, board-certified obstetrician/gynecologist with 19 years of clinical practice, trained at The Ohio State University.
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Due Date Is Calculated
Your due date is an estimate of when you'll reach 40 weeks of pregnancy, calculated from whichever dating information you have available. ACOG recommends using the earliest, most reliable data on hand, since dating accuracy generally improves the earlier in pregnancy it's established. Here's exactly how each method works.
What This Calculator Measures
This calculator estimates your due date from one of three starting points: your last menstrual period, a known conception date, or an early ultrasound. It does not predict your actual delivery date, which naturally varies by several weeks even in healthy pregnancies.
The Formula
From LMP: Due date = LMP + 280 days
From conception date: Due date = Conception date + 266 days
From ultrasound: Due date = Ultrasound date + (280 minus gestational age in days at the scan)
All three formulas trace back to Naegele's Rule, a 19th-century method still used in clinical dating today¹. The conception and ultrasound formulas are direct mathematical extensions of the same 280-day estimate.
How This Calculator Computes Your Result
Pick whichever starting point you have: LMP with your average cycle length, a known conception date, or an ultrasound date with gestational age at the scan. If your cycle isn't exactly 28 days, we adjust the LMP-based estimate to account for the difference. Each path runs its own formula above and returns a single due date.
What Your Result Means
Term Window | Weeks |
Early Term | 37 weeks to 38 weeks, 6 days |
Full Term | 39 weeks to 40 weeks, 6 days |
Late Term | 41 weeks to 41 weeks, 6 days |
Post-Term | 42 weeks and beyond |
Your due date is the center of this range, not a deadline. ACOG defines these term windows because babies born at different points within them have measurably different average outcomes¹.
Cross-Checking When Multiple Dates Are Available
If you enter both an LMP and an ultrasound date and they disagree, we flag it rather than silently picking one. Per ACOG guidance, an early ultrasound should generally take precedence over LMP-based dating when the two disagree meaningfully, since first-trimester ultrasound is the most accurate dating method available¹, a finding also supported by direct comparisons of ultrasound and menstrual dating³.
A Note on Accuracy
Only about 4 to 5% of babies are born exactly on their estimated due date², and pregnancy length naturally varies by several weeks even without complications. LMP-based dating assumes a 28-day cycle with ovulation on day 14, which doesn't hold for everyone. Treat your due date as the center of a range, not a deadline.
Frequently Asked Questions
How is my due date calculated?
From your last period, due date equals LMP plus 280 days, adjusted for cycle length¹. From a known conception date, we add 266 days, and from an ultrasound, we use the scan date plus the remaining days to reach 280 total.
Is ultrasound dating more accurate than my last period?
Yes, especially in the first trimester. Ultrasound measurements of the embryo are generally more accurate than LMP-based dating alone, particularly for anyone with irregular cycles¹, a finding also borne out by direct comparisons of the two methods³.
Will I actually deliver on my due date?
Probably not exactly. Only about 4 to 5% of babies arrive on their estimated due date, though most arrive within about two weeks of it either way².
What do early term, full term, late term, and post-term mean?
ACOG's official windows relative to your due date¹: early term is 37 to 38 weeks, full term is 39 to 40 weeks, late term is 41 weeks, and post-term is 42 weeks or beyond.
Sources
Methods for Estimating the Due Date, ACOG Committee Opinion No. 700, 2017
Length of Human Pregnancy and Contributors to Its Natural Variation, Jukic et al., 2013
Estimating the Date of Confinement: Ultrasonographic Biometry Versus Certain Menstrual Dates, Mongelli et al., 1996