Fetal Growth & Pregnancy Date Calculator
Use your last menstrual period (LMP) to estimate gestational age, estimated due date, conception window, trimester, and fetal age.
What is a calculator fetal tool?
A calculator fetal tool is designed to estimate common pregnancy timeline milestones from a small set of inputs, usually your last menstrual period (LMP). It helps you answer practical questions such as:
- How many weeks pregnant am I today?
- What is my estimated due date (EDD)?
- Which trimester am I in right now?
- Roughly when did conception occur?
These estimates are very useful for planning appointments, understanding fetal development stages, and keeping track of pregnancy progress in a simple way.
How this fetal calculator works
1) Gestational age
Gestational age counts from the first day of your LMP, not from conception. This is the standard method used in obstetrics. The calculator computes the number of days between your LMP and the reference date, then converts that into weeks and days.
2) Estimated due date (EDD)
The classic due date estimate is LMP + 280 days (40 weeks). If your cycle is longer or shorter than 28 days, this page adjusts the estimate by the cycle-length difference.
3) Conception window
Conception often occurs near ovulation. For a 28-day cycle, ovulation is commonly estimated around day 14. For other cycle lengths, ovulation shifts accordingly. This tool gives a practical 5-day conception window centered on that estimate.
4) Trimester classification
- First trimester: 0 weeks to 13 weeks, 6 days
- Second trimester: 14 weeks to 27 weeks, 6 days
- Third trimester: 28 weeks onward
Why estimated dates can differ from ultrasound dates
Pregnancy dating can vary for many reasons. Ovulation does not happen on the exact same day in every cycle, implantation timing differs, and cycle length can change month-to-month. Early ultrasound measurements are often used clinically to refine dating because they can provide a more direct developmental reference.
If your ultrasound date differs from your LMP estimate, follow your provider’s guidance—they may use specific medical criteria to assign the most accurate due date.
Best practices when using pregnancy calculators
- Use the most accurate LMP date you can remember.
- Set your average cycle length honestly, especially if it is not 28 days.
- Update your reference date to track week-by-week progress.
- Treat all output as estimates, not diagnosis.
- Confirm important decisions with your OB-GYN or midwife.
Common questions
Is fetal age the same as gestational age?
No. Gestational age starts at LMP. Fetal age starts near conception, usually about 2 weeks later in a 28-day cycle. That is why fetal age is often lower than gestational age.
Can this calculator diagnose fetal health?
No. This tool does not assess fetal well-being, anomalies, or risk conditions. It only estimates timeline dates.
What if my cycles are irregular?
For irregular cycles, calculator estimates are less precise. In those cases, ultrasound-based dating and direct prenatal care are especially important.
Final note
A calculator fetal page is a helpful planning companion for pregnancy tracking, especially in early pregnancy when parents are trying to map milestones. Use it as a guide, keep your prenatal appointments, and rely on your medical team for personalized care.