Pregnancy Due Date & Timeline Calculator Ireland

How pregnancy dating works

Pregnancy is conventionally dated from the first day of the last menstrual period (LMP) rather than from the date of conception. The standard LMP calculation assumes a pregnancy length of 280 days, or 40 weeks.

That convention also assumes a regular 28-day menstrual cycle with ovulation around day 14. Real cycles and ovulation dates vary, and people may not recall the LMP exactly. That is why a calendar calculation is an estimate rather than a clinical measurement.

Basic and Advanced modes

Basic uses the conventional 280-day estimate from the first day of the LMP. Advanced is useful when your cycle is not 28 days or your maternity team has already assigned a due date. A clinician-assigned due date takes priority over the LMP calculation.

If no assigned due date is entered, Advanced can shift the LMP-based estimate by the difference between your typical cycle length and 28 days. This is still a calendar estimate. It does not override ultrasound dating or the due date recorded by your maternity team.

The progress bar is simply elapsed calendar time relative to a 40-week timeline. It is not a measure of fetal growth, viability or how the pregnancy is progressing clinically.

Ultrasound dating is more accurate early in pregnancy

The American College of Obstetricians and Gynecologists, together with the American Institute of Ultrasound in Medicine and the Society for Maternal–Fetal Medicine, states that first-trimester ultrasound up to 13 weeks and 6 days is the most accurate method to establish or confirm gestational age. Crown–rump-length dating in this period is typically accurate to about ±5–7 days.

HSE says all pregnant women in Ireland are offered a dating scan and that this should be done before 15 weeks. The standard LMP method assumes a 28-day cycle with ovulation around day 14, so later or earlier ovulation, irregular cycles or uncertain menstrual dates can make the scan and LMP estimates differ.

If your maternity team has already assigned a due date from the clinical record or dating scan, enter that in the optional field. Compound then uses that date for the timeline rather than trying to override it with an LMP calculation. Once an EDD has been established, later scans should not be used casually to keep changing the date because later fetal size increasingly reflects growth as well as gestational age.

The HSE’s own pregnancy calculator similarly describes its result as an estimate based on a 40-week pregnancy. ACOG notes that only about 1 in 20 women give birth on the exact estimated due date, so the EDD should be understood as a clinical timeline anchor rather than a prediction of the exact birth day.

What the conception date means

The conception date displayed here is not a claim about the exact day fertilisation occurred. It is simply the date 266 days before the due date — equivalent to approximately two weeks after LMP in the conventional 28-day model.

Ovulation varies, sperm can survive for several days, and pregnancy dating is deliberately based on gestational age rather than an attempt to identify a precise conception date.

Timeline milestones

The tool shows calendar points that are useful for orientation:

• 12 weeks — a simple gestational milestone, not a guaranteed appointment date.
• 18 to 22 weeks — the HSE says the fetal anatomy/anomaly scan is usually offered in this window.
• 37 weeks — the beginning of the early-term period in common obstetric terminology.
• 40 weeks — the estimated due date.
• 42 weeks — a calendar milestone two weeks after the due date.

Your actual appointments and management should follow your maternity team’s plan, not a website timeline.

Important limitations

This calculator cannot assess pregnancy viability, fetal growth, scan findings, bleeding, pain, reduced movements or any other clinical concern. A due date is also not a prediction of the exact day a baby will be born.

If the dates here differ from the dates given by your maternity team, use the dates in your clinical record and ask your midwife, GP or obstetric team if you are unsure.

Worked example

If the first day of the last menstrual period was 1 January 2026, the standard 280-day calculation gives an estimated due date of 8 October 2026.

On that same LMP-based timeline:

• 12 weeks falls around 26 March 2026;
• the calculator’s 18–22 week anatomy-scan window runs roughly 7 May to 4 June 2026;
• 37 weeks is around 17 September 2026.

Those are calendar landmarks, not promises about when an individual scan, test or birth will occur. If a maternity team assigns a different due date after clinical dating, the assigned date should take priority for care planning.

Use one agreed clinical date for planning

It is common for an LMP-based estimate and an early ultrasound estimate to differ slightly. For appointments, screening windows and maternity planning, use the estimated due date assigned by the maternity team rather than switching between calculator outputs.

The calculator remains useful for orientation: it can translate the agreed due date into an approximate gestational timeline and show where major calendar milestones fall.

If cycle length is unusual or the last menstrual period is uncertain, the LMP estimate is inherently less reliable. That is one reason early ultrasound dating can become the preferred clinical reference.

For practical household planning rather than medical decisions, pair the timeline with the Childcare Return-to-Work Calculator when considering future childcare and employment costs.

Method and limitations

LMP dating adds 280 days to the first day of the last menstrual period. The conception estimate is calculated 266 days before the due date, which is only an approximation. If an assigned due date is entered, Compound works backwards 280 days to create the displayed gestational timeline.

Cycle length, ovulation timing and uncertain LMP dates can make menstrual dating inaccurate. Early ultrasound is generally more reliable for establishing gestational age when dates are uncertain. This tool is for timeline orientation and does not replace maternity care.

Related planning tools

The pregnancy timeline is intentionally kept separate from financial or nutrition advice. For later household planning, the Childcare Cost vs Returning to Work Calculator can model childcare and employment cash flow, while the Lifetime Cost Calculator can be used for broader long-term household scenarios.

Neither tool changes or interprets the clinical pregnancy dates shown here.

Frequently asked questions

Is the estimated conception date exact?

No. It is an approximate point derived from the due date. Ovulation and implantation vary, and intercourse that leads to conception can occur several days before ovulation.

Why can my scan change the due date?

Early ultrasound estimates gestational age from fetal measurements and can be more reliable than menstrual dating when cycle length or LMP timing is uncertain.

Does a due date mean the baby will arrive that day?

No. It is an estimated reference date. Normal birth timing spans a range around it.

Should I replace the date my maternity team gave me?

No. If your clinician has assigned an estimated due date, use that clinical date for care planning.

Related Compound tools

The calculator is designed for timeline orientation rather than medical decision-making. For pregnancy symptoms, screening, medication questions or concerns about fetal growth, use your GP, midwife, obstetric team or HSE maternity guidance.