Gestational Age Calculator – Current Weeks and Due Date
The Gestational Age Calculator determines your current gestational age and estimated due date using either the last menstrual period (LMP) date or ultrasound measurements. Enter your last period date, ultrasound date, and ultrasound-confirmed gestational age in weeks and days — and get your current gestational age in weeks and days, estimated due date, estimated conception date, and trimester information. Useful for expectant mothers tracking pregnancy milestones, medical professionals verifying gestational dating, and IVF patients cross-referencing clinical and LMP-based dating. For personalised advice, consult a qualified obstetrician.
Formula
This calculator transforms the provided inputs into the requested outputs using standard domain equations.
Quick Tip
Use this output as guidance and confirm clinical decisions with a qualified professional.
Want to know exactly how many weeks pregnant you are right now — and your due date? Enter your last period date or ultrasound details and get your current gestational age and full trimester timeline instantly.
Featured Answer
Q: How do I calculate gestational age from my last period?
A: Gestational age is calculated by counting days from the first day of the last menstrual period (LMP) to today, then converting to weeks and days. For example, if the LMP was 1 January 2026, on 1 April 2026 the gestational age is 13 weeks exactly. The estimated due date is LMP + 280 days (40 weeks). Use this calculator to get your current gestational age and due date instantly.
How to Use Gestational Age Calculator
- Enter your last period date — the first day of your most recent menstrual period before pregnancy.
- Enter the ultrasound date — the date of your dating scan, if available.
- Enter the ultrasound gestational age in weeks and days — the gestational age confirmed by the sonographer at that scan date.
What is Gestational Age?
Gestational age is the duration of a pregnancy measured from the first day of the mother's last menstrual period (LMP), expressed in completed weeks and days. It is the standard unit of pregnancy measurement in obstetrics worldwide.
Gestational age is approximately 2 weeks longer than embryonic age (counted from conception) because it starts from the LMP, which precedes ovulation and fertilisation by approximately 14 days in a 28-day cycle.
The standard full-term pregnancy is 40 weeks gestational age from LMP — equivalent to approximately 38 weeks from conception.
LMP-based dating is the simplest method but depends on a regular 28-day cycle. Ultrasound dating is more accurate, especially when performed before 12 weeks, as it measures the embryo or fetus directly.
This calculator uses both inputs and indicates which is more reliable for dating purposes. When ultrasound and LMP dates disagree by more than 5–7 days (first trimester) or 10–14 days (second trimester), the ultrasound dating typically takes precedence clinically.
Example: LMP: 5 January 2026. Today: 11 April 2026.
| Field | Value |
|---|---|
| Gestational Age | 14 weeks 0 days |
| Estimated Due Date | 12 October 2026 |
| Conception Estimate | ~19 January 2026 |
| Current Trimester | Second trimester |
| Trimester 2 Ends | ~28 July 2026 |
Gestational Age: How Pregnancy Weeks Are Counted and Why It Matters
Why Gestational Age Calculator Matters
Knowing your exact gestational age is not just academic — it drives almost every clinical decision in prenatal care. Which screening tests to schedule and when. Whether a baby's growth is on track. When to start monitoring for preterm labour. When to consider delivery if complications arise.
For expectant mothers, gestational age answers the most common questions: how far along am I, and when is my due date? This calculator gives both in seconds, using either LMP or ultrasound data — or both together for cross-validation.
By the way — ultrasound dating before 12 weeks is considered the most accurate method, within plus or minus 5 days. LMP-based dating assumes a regular 28-day cycle and ovulation on Day 14, which does not apply to everyone. This tool handles both.
How Gestational Age Is Calculated — Step by Step
LMP method:
- Count days from LMP to today.
- Divide by 7 for complete weeks; remainder is days.
- Due date = LMP + 280 days.
Ultrasound method:
- From the ultrasound gestational age (in weeks and days) at the scan date, calculate the LMP-equivalent start date: scan date minus gestational age in days.
- From this adjusted start date, count forward to today for current gestational age.
- Due date = adjusted start date + 280 days.
When both methods are available: compare the two due dates. If within 5 days (1st trimester) or 10 days (2nd trimester), either is acceptable. If they differ more, ultrasound dating takes precedence.
Gestational Age Milestones
| Gestational Age | Milestone |
|---|---|
| 6 weeks | Fetal heartbeat detectable on transvaginal ultrasound |
| 10–13 weeks | First trimester screening window (nuchal translucency) |
| 12 weeks | End of first trimester |
| 18–20 weeks | Anomaly scan (detailed anatomy ultrasound) |
| 24 weeks | Viability threshold |
| 28 weeks | Third trimester begins |
| 37 weeks | Term — safe delivery range begins |
| 40 weeks | Estimated due date |
| 42 weeks | Post-term — labour typically induced |
Common Mistakes to Avoid
- Using cycle day 1 vs LMP day 1 interchangeably — LMP day 1 is the first day of bleeding in the last period, which is the standard obstetric reference point. Using a different day introduces dating error.
- Assuming gestational age equals months pregnant — gestational weeks and calendar months do not map neatly. 40 weeks is approximately 9 calendar months plus 1 week. People often overestimate their month of pregnancy when counting by calendar.
- Not updating the gestational age calculation after a scan revision — if a first-trimester scan changes the due date by 5+ days, the new date supersedes the LMP-based calculation. Recalculate from the scan date.
- Confusing gestational age with embryonic age — embryonic age (from conception) is approximately 2 weeks less than gestational age. Most pregnancy apps report gestational age; some embryology references use embryonic age.
When to Use This Calculator
Use this tool at any point during pregnancy to confirm gestational age, cross-check against a scan result, or establish the due date from a known LMP. Also useful for tracking progress toward specific milestones — when to schedule the anomaly scan, when to expect the third trimester to begin.
For IVF-specific gestational dating based on embryo transfer date, the IVF Due Date Calculator provides the more accurate transfer-based calculation. For conception date estimation from a known due date, the Conception Calculator works in the reverse direction.
Important Assumptions and Limitations
LMP-based calculation assumes a regular 28-day cycle with ovulation on Day 14. Irregular cycles reduce LMP-based dating accuracy. Ultrasound dating accuracy is highest before 12 weeks (±5 days) and decreases in later pregnancy (±2–3 weeks by the third trimester). Calculation reviewed against standard obstetric gestational dating references.
For personalised advice, consult a qualified obstetrician.
Frequently Asked Questions
Find answers to common questions about Gestational Age Calculator
Gestational age is the duration of a pregnancy measured from the first day of the mother's last menstrual period (LMP), expressed in weeks and days. Full-term pregnancy is 40 weeks gestational age. It differs from embryonic age by approximately 2 weeks, as it starts from LMP rather than the actual conception date. Gestational age is the universal standard in obstetric clinical practice.
Count the number of days from the first day of your last menstrual period to today. Divide by 7 to get complete weeks; the remainder is the additional days. Add 280 days to the LMP date for the estimated due date. For example, an LMP of 1 January and a current date of 1 April gives 90 days = 12 weeks and 6 days gestational age.
LMP-based calculations are accurate for women with regular 28-day cycles, typically within 5–7 days. For irregular cycles, LMP dating is less reliable. Ultrasound dating before 12 weeks is the most accurate method (±5 days) and is used clinically when available. This calculator uses whichever method is entered and indicates when both methods are available and whether they agree.
Trimester information shows which of the three pregnancy trimesters corresponds to the current gestational age — first (weeks 1–12), second (weeks 13–27), or third (weeks 28–40) — and the dates on which the current trimester began and ends. This helps with scheduling prenatal care, understanding what developmental stage the pregnancy is at, and anticipating what screening tests are due.
Ultrasound dating before 12 weeks is the gold standard — accurate within approximately 5 days. Between 12 and 20 weeks, accuracy is within 7–10 days. After 28 weeks, individual fetal size variation is too large for reliable gestational age dating from measurements alone. An LMP date combined with a first-trimester scan provides the most reliable gestational age for clinical management.
Gestational age is counted from the LMP — approximately 2 weeks before conception. Fetal (or embryonic) age is counted from the actual conception date. For a pregnancy at 12 weeks gestational age, the fetus is approximately 10 weeks old in embryonic terms. Gestational age is universally used in clinical obstetrics; embryonic age appears in developmental biology and IVF literature.
Yes — but for irregular cycles, prioritise the ultrasound-based dating over the LMP calculation. Enter the LMP date if known, but also enter your first-trimester ultrasound date and confirmed gestational age. The ultrasound result is far more reliable for gestational dating when cycle length varies. If you only have an LMP date and an irregular cycle, treat the LMP-based result as an estimate.
Ultrasound gestational age is measured directly from fetal size (crown-rump length in the first trimester, biometric measurements later). LMP gestational age assumes a 28-day cycle with Day-14 ovulation. When they agree (within 5–7 days), LMP is confirmed. When they disagree, the ultrasound is used for clinical management — particularly if the first-trimester scan shows the fetus is smaller or larger than LMP dating would predict.