Gestational Age Calculator - Calculate Pregnancy Week & Fetal Development
Estimate your pregnancy week, trimester, and due date from your LMP, conception date, or an ultrasound dating scan.
Choose the dating method you know best, enter the relevant dates, and the calculator will estimate your current gestational age, week of pregnancy, and expected due date.
Gestational Age Calculator - Calculate Pregnancy Week & Fetal Development
Estimate your pregnancy week, trimester, and due date from your LMP, conception date, or an ultrasound dating scan.
About the gestational age calculator
Gestational age is the standard measurement of how far a pregnancy has progressed, counted in weeks and days from the first day of the last menstrual period (LMP). This convention became standard long before home pregnancy tests existed, because the LMP is a date women can usually remember precisely, while the date of ovulation and conception can be harder to pinpoint. As a result, the full-term length of a pregnancy is conventionally quoted as 40 weeks from LMP, even though fertilisation typically occurs about two weeks after the LMP.
There are three common methods for calculating gestational age. The LMP method uses the date of your last period along with your average cycle length. The standard Naegele's Rule assumes a 28-day cycle and adds 280 days to the LMP to arrive at the estimated due date. For cycles longer or shorter than 28 days, the due date shifts by the number of days the cycle differs from 28. This calculator adjusts automatically when you enter your cycle length.
The conception date method is useful when the date of conception is known with confidence — for example, after assisted reproduction or close monitoring of ovulation. Because fertilisation happens roughly 14 days after the LMP in a standard cycle, gestational age from conception is calculated by adding 14 days to the days since conception. The due date becomes 266 days from conception, which equals 280 days from the corresponding LMP.
Ultrasound dating is performed by a clinician and uses measurements of the embryo or fetus — crown-rump length in the first trimester, or head circumference, femur length, and abdominal circumference later on — to estimate gestational age. First-trimester ultrasound (before 14 weeks) is the most accurate, typically within plus or minus 5 to 7 days. Later scans have wider margins of error. This calculator lets you enter the gestational age recorded at your ultrasound appointment and adds the days that have passed since then to give your current gestational age.
Pregnancy is divided into three trimesters. The first trimester spans weeks 1 through 13, covering implantation, embryonic organ formation, and the highest period of miscarriage risk. The second trimester covers weeks 14 through 27, often described as the most comfortable phase when many symptoms ease and fetal movement begins. The third trimester runs from week 28 to delivery, a phase of rapid fetal growth and lung maturation. Delivery between 37 and 42 weeks is considered term; before 37 weeks is preterm, and after 42 weeks is post-term.
This calculator is an educational tool and provides estimates only. Due dates are approximate — fewer than 5 percent of babies arrive on their estimated due date, and a normal delivery range extends from 37 to 42 weeks. Always discuss your gestational age and pregnancy timeline with your midwife, obstetrician, or other qualified healthcare professional.
Gestational age calculation examples
These examples show how the three dating methods work and what gestational age means at different stages.
| Input | Result | Notes |
|---|---|---|
| LMP 10 weeks ago, 28-day cycle | 10w 0d, Due ~30 weeks away | Standard LMP calculation using Naegele's Rule. Second trimester begins at week 14. |
| Conception date 8 weeks ago | 10w 0d gestational age (adds 14 days), Due ~30 weeks away | Conception method adds 14 days to align with LMP-based gestational age convention. |
| Ultrasound 2 weeks ago showed 8w 3d | 10w 3d gestational age today | Ultrasound dating is most accurate in the first trimester (within ±5–7 days). |
How to use the gestational age calculator
- Select your calculation method: LMP if you know your last period date, Conception Date if you know when conception occurred, or Ultrasound Dating if you have a scan result.
- Enter the relevant date in the date field — your LMP date, conception date, or the date of the ultrasound scan.
- If using LMP, confirm or adjust your average cycle length (default is 28 days). For ultrasound, enter the gestational age shown on the scan report in weeks and days.
- Click Calculate to see your current gestational age in weeks and days, the current pregnancy week, estimated due date, and trimester.
- Use Reset to start over, or switch between calculation methods to compare results from different dating approaches.
Gestational age calculator FAQ
Why is gestational age counted from the last period rather than conception?
Gestational age has traditionally been measured from the first day of the last menstrual period because it is a date most women know, whereas the exact day of ovulation and conception is harder to confirm. In practice this means gestational age is about two weeks ahead of fetal age from conception.
How accurate is the LMP method?
The LMP method is accurate when you have a regular cycle and remember the date precisely. For women with irregular cycles, longer cycles, or uncertain LMP dates, ultrasound dating — especially a first-trimester scan — is more reliable and will often be used to revise the due date.
What does my due date actually mean?
Your estimated due date (EDD) is a statistical midpoint, not an exact prediction. Normal pregnancies range from 37 to 42 weeks of gestation. Only about 4–5 percent of babies arrive on the calculated EDD; most deliver within the two weeks before or two weeks after it.
When does the third trimester start and why does it matter?
The third trimester begins at week 28 and continues until delivery. During this phase the fetus gains most of its birth weight, lungs mature, and babies born at 28 weeks or later have increasingly good survival rates with modern neonatal care. Understanding your trimester helps you and your care team track appropriate milestones and prepare for delivery.
Why does my cycle length affect the due date calculation?
Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, ovulation and conception happen later in the cycle, which shifts the due date forward. If your cycle is shorter, the due date moves earlier. Adjusting for cycle length improves the accuracy of LMP-based dating.
Is this calculator a substitute for prenatal care?
No. This tool is for educational and planning purposes only. Gestational age confirmed by a clinician, especially with ultrasound, is more accurate. Always work with a qualified midwife, obstetrician, or family physician for all medical decisions about your pregnancy.