Gestational Age Calculator (Based on LMP)
Use this tool to estimate your current gestational age and due date from the first day of your last menstrual period (LMP).
For educational use only. Always confirm dating and due date with your OB-GYN, midwife, or prenatal care team.
How this gestational age calculator works
This calculator uses a standard obstetric method: counting pregnancy length from the first day of your LMP. Clinically, this is called gestational age. It is measured in weeks and days (for example, 10 weeks 3 days).
Most pregnancy timelines are based on a 280-day (40-week) model from LMP for a 28-day cycle. If your average cycle is longer or shorter, the estimated due date can shift slightly, and this calculator adjusts for that.
What you get in the results
- Estimated gestational age: Weeks + days pregnant on your selected date.
- Estimated due date (EDD): Your projected delivery date based on LMP and cycle length.
- Estimated conception date: Approximate ovulation/conception timing.
- Trimester: First, second, or third trimester stage.
- Time remaining: Approximate days/weeks until due date.
LMP dating formula (simple version)
For a 28-day cycle
The classic method is Naegele's rule: add 280 days (40 weeks) to the first day of your LMP.
For non-28-day cycles
If your cycle differs from 28 days, due date is adjusted by the cycle difference:
- Cycle longer than 28 days: due date tends to be later
- Cycle shorter than 28 days: due date tends to be earlier
Why gestational age and fetal age are different
Gestational age starts at LMP, which is about two weeks before ovulation in a typical cycle. So fetal age (time since conception) is usually about 2 weeks less than gestational age. Both terms are useful, but prenatal clinics usually use gestational age.
When this estimate may be less accurate
- Irregular menstrual cycles
- Uncertain LMP date
- Recent hormonal contraception use
- Breastfeeding with irregular cycles
- Recent miscarriage or postpartum cycle changes
In these situations, a first-trimester ultrasound often provides more precise pregnancy dating.
When to contact your prenatal provider
If your calculated dates are very different from ultrasound findings, or if your cycles are highly variable, ask your provider to confirm your official due date. Your care plan, screening windows, and timing of tests all rely on accurate dating.
Frequently asked questions
Is this the same as a due date calculator?
Yes. A gestational age calculator from LMP and a due date calculator are closely related tools. One gives your current pregnancy age; the other gives your expected due date.
Can I use this if I conceived via IVF?
IVF pregnancies are typically dated using embryo transfer date and embryo age. Use IVF-specific dating guidance from your fertility or obstetric team.
What is full term?
Full term is generally considered 39 weeks 0 days through 40 weeks 6 days. However, individual delivery timing varies, and many births occur before or after the exact due date.
Important note
This online calculator is an estimate and does not replace medical care. For diagnosis, treatment decisions, and official pregnancy dating, please consult a qualified healthcare professional.