Longer cycles usually move the estimated due date later, while shorter cycles move it earlier.
Clinical context
Use the last-period method for standard pregnancy dating, conception mode when you know conception timing with confidence, and dating scan mode when an ultrasound gave you a gestational age.
Known due date mode works backwards from an EDD when you want gestational age and milestone context from a date your clinician already gave you.
IVF or embryo-transfer pregnancies use a different timing method, so that path is better handled with the separate IVF due date calculator.
Estimated due date
1 Jan 2027
224 days until the estimated due date.
Current gestational age
8w 0d
Trimester
First trimester
Dating basis
Last menstrual period
Days to due date
224
Estimated timeline
Last menstrual period: 27 Mar 2026
Estimated conception: 10 Apr 2026
Planning milestones
End of first trimester: 2 Jul 2026
20-week mark: 14 Aug 2026
37-week mark: 11 Dec 2026
39-week mark: 25 Dec 2026
41-week mark: 8 Jan 2027
42-week mark: 15 Jan 2027
Clinical confirmation matters If your dates are uncertain, cycles are irregular, symptoms are unusual, or an ultrasound gives a different estimate, confirm pregnancy timing with your midwife, obstetrician, or GP.
Due date calculator guide: estimate pregnancy due date, weeks pregnant, and dating methods
A due date calculator helps you estimate when a baby may be due, how many weeks pregnant you are today, and which dating method best fits the information you actually know. This version works from last menstrual period, a known conception date, or an ultrasound dating scan, then turns the estimate into gestational age, trimester, and practical milestone dates you can discuss with your maternity team.
What this due date calculator helps you work out
A good due date calculator does more than show one calendar date. It helps you translate the information you know now into an estimated due date, current gestational age, trimester, and a few practical markers that make pregnancy timing easier to follow.
That matters because different people start with different evidence. Some know the first day of the last period, some know conception timing, and others are working from an early dating scan. A calculator that recognises those different starting points is closer to how pregnancy dating works in real care.
How LMP, conception, and ultrasound dating differ
The standard calendar method starts from the first day of the last menstrual period, often shortened to LMP. By convention, pregnancy is dated as 280 days, or 40 weeks, from that date. If you know conception timing instead, the equivalent estimate is usually about 266 days from conception because gestational dating starts roughly two weeks earlier than fertilisation.
Ultrasound dating works differently. Instead of assuming ovulation timing from a cycle, it uses the gestational age already estimated at the scan and projects forward to the 40-week point. That is why early ultrasound can revise the calendar estimate when LMP is uncertain, cycles are irregular, or scan measurements differ meaningfully from the date-based estimate.
Estimated due date = LMP + 280 days
This is the conventional 40-week rule used by many due date and pregnancy weeks calculators.
Estimated due date = Conception date + 266 days
This is the conception-based equivalent, because conception usually occurs about two weeks after the LMP date in a 28-day cycle.
Gestational age = Current date - LMP
Gestational age is usually tracked in completed weeks and days from the LMP-based start of pregnancy dating.
Estimated due date = Scan date + (280 days - gestational age at scan)
This is the ultrasound-dating equivalent used when a scan already gives a gestational age in completed weeks and days.
Why cycle length changes an LMP-based estimate
If your cycle is longer or shorter than 28 days, the LMP-based estimate shifts because ovulation is unlikely to happen on the same day in every cycle. The calculator therefore lets you adjust cycle length instead of hard-coding a 28-day assumption for every user.
A longer cycle usually moves the due date later, and a shorter cycle usually moves it earlier. That is one reason people searching for a due date calculator, pregnancy due date calculator, or estimated due date calculator may see different answers across tools when they enter the same last period date.
Adjusted LMP due date = LMP + 280 days + (cycle length - 28 days)
The LMP path shifts by the number of days your average cycle differs from the conventional 28-day cycle.
When you already know the estimated due date
Many people arrive with an estimated due date already written in a scan report, maternity app, referral letter, or clinic note. In that situation, the useful question is often not how to find the due date again, but how to translate that known EDD into weeks pregnant, days until the due date, the LMP-equivalent date, and the term-window milestones.
The known due date mode works backwards from the EDD by 280 days to estimate the LMP-equivalent starting point, then uses that anchor to show gestational age and planning dates. This makes the page work as both a pregnancy due date calculator and a quick pregnancy weeks calculator without pretending that a web estimate overrides the date assigned by your maternity team.
LMP equivalent = Known EDD - 280 days
This back-calculates the gestational dating anchor from an existing estimated due date.
Estimated conception = Known EDD - 266 days
This gives the conception-timing equivalent under the standard 40-week pregnancy dating convention.
Worked example: 8-week dating scan on 1 March 2026
Suppose an ultrasound on 1 March 2026 measures the pregnancy at 8 weeks 0 days. A due date calculator can work backwards to an estimated LMP around 4 January 2026, then project forward to an estimated due date of 11 October 2026. The same result is then converted into current gestational age, trimester, and milestone dates such as the 20-week mark.
This is useful because it turns the scan result into a clearer planning picture, but it is still not a substitute for clinical review. If a sonographer or obstetrician gives you a dating estimate, that professional assessment should take priority over any web tool.
Why the estimate can change after your first appointment
A due date is an estimate rather than a guarantee. People do not all ovulate on day 14, cycles are not always 28 days long, and the first day of the last period may not be remembered exactly. This is why a date-based due date can shift after a first-trimester scan.
In practice, early ultrasound often becomes the more reliable dating anchor when the calendar history is uncertain or when scan measurements differ materially from the LMP estimate. That is normal pregnancy care, not a sign that something is wrong.
What this page does not cover
This page does not replace IVF-specific dating, detailed ultrasound biometry, or clinician-led review of bleeding, pain, reduced movement, or uncertainty about how far along the pregnancy is. If you conceived through IVF, use the dedicated IVF due date calculator because embryo-transfer timing uses a different dating method.
These figures are for general planning only. If you are pregnant or think you may be pregnant, confirm dates and next steps with your midwife, obstetrician, or GP, especially if cycles are irregular, the last period is uncertain, or an ultrasound gives a different estimate.
NHS — Due date calculator — Public guidance on calculating an estimated due date from the first day of the last period and cycle length.
ACOG — When Pregnancy Goes Past Your Due Date — Patient-facing explanation that the average pregnancy lasts about 280 days from the first day of the last menstrual period and why due dates are estimates.
Frequently asked questions
How is the estimated due date calculated?
The standard method adds 280 days or 40 weeks to the first day of the last menstrual period. If you know conception timing instead, the estimate is usually 266 days from conception. When an ultrasound already gives gestational age at scan, the due date can be projected forward to the 40-week point from that measurement.
Why can my due date change after an ultrasound?
Because an early dating scan can measure gestational age more accurately than memory of cycle dates alone. If the last period is uncertain, cycles are irregular, or the scan measurement differs meaningfully from the calendar estimate, clinicians may revise the expected due date. That adjustment is part of normal pregnancy dating care.
What if I conceived through IVF or embryo transfer?
IVF pregnancies use transfer-date and embryo-age timing rather than standard ovulation assumptions. That is why IVF due date tools count from the embryo transfer and embryo age instead of using a generic LMP or conception rule. If IVF is relevant, use an IVF-specific calculator or your fertility clinic's dating instructions.
How often are babies born exactly on their due date?
Only a small minority of babies arrive on the exact estimated due date. The due date is a planning anchor and clinical reference point, not a guarantee of delivery on one specific day. That is why clinicians talk about ranges such as term or post-term rather than treating one date as fixed.
Does cycle length change the estimated due date?
Yes. Longer cycles usually move the LMP-based estimate later, while shorter cycles move it earlier. That is why the calculator asks for average cycle length when you use the last-period method.
Can I use the calculator if I already know my due date?
Yes. Choose known due date mode and enter the EDD from your scan report or clinician. The calculator will work backwards to an LMP-equivalent date, then show how many weeks pregnant that date implies, how many days remain until the due date, and when the 37-, 39-, 41-, and 42-week milestones fall.
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