About the Pregnancy Calculator
A pregnancy calculator estimates gestational age — how many weeks and days pregnant you are — most commonly based on the first day of your last menstrual period (LMP). Gestational age is the standard medical clock for pregnancy: it determines the expected due date, frames every screening test, and structures the schedule of prenatal visits. Note: this tool is for general information; an obstetric provider's dating, often confirmed by first-trimester ultrasound, is what should drive medical decisions.
How pregnancy is dated
By medical convention, gestational age is measured from the first day of the last menstrual period (LMP), not from conception. This adds about two weeks compared to the actual time since fertilization, but it's the universal medical standard because LMP is something the patient typically knows and conception is rarely datable to a specific day. A full-term pregnancy is conventionally 40 weeks of gestational age, with normal range 37–42 weeks.
First-trimester ultrasound (typically 8–13 weeks) is the most accurate way to date a pregnancy and is recommended by ACOG when LMP-based dating is uncertain or differs significantly from ultrasound dating. After the first trimester, ultrasound dating becomes progressively less precise.
The trimesters and what defines them
First trimester: weeks 1–12 (or 1–13). Embryonic development, organ formation, and the highest miscarriage risk are concentrated here. Pregnancy symptoms — nausea, fatigue, breast tenderness — typically peak in the first trimester. Most non-elective miscarriages occur before 12 weeks, and risk drops sharply after a confirmed fetal heartbeat.
Second trimester: weeks 13–27. Often the most comfortable stretch — first-trimester symptoms ease, the pregnancy is visibly evident by ~16–20 weeks, and fetal movement ("quickening") begins around 18–22 weeks for first pregnancies. The anatomy ultrasound at 18–22 weeks evaluates fetal development in detail.
Third trimester: weeks 28–40+. Rapid fetal growth, maternal physiological adaptations (cardiac output, blood volume, joint laxity) peak. The schedule of prenatal visits intensifies — typically every 2 weeks from 28–36 weeks, then weekly until delivery.
Naegele's rule and the actual distribution of due dates
Naegele's rule estimates the due date by adding 280 days (40 weeks) to the first day of LMP, or equivalently subtracting 3 months and adding 7 days. It assumes a 28-day cycle with ovulation on day 14; for women with longer or shorter cycles, the calculation should be adjusted accordingly.
Only about 4% of pregnancies actually deliver on the calculated due date. The full-term range (37–42 weeks) covers about 88% of births. Researchers have noted that the average gestation length appears to be slightly longer than 280 days for first-time mothers — closer to 285–288 days — though these averages vary across populations and study methodologies.
When the calculator's date and the ultrasound date disagree
If LMP-based dating differs from first-trimester ultrasound dating by more than 5–7 days (in early gestation), professional guidelines (ACOG/AIUM) generally recommend using the ultrasound date. The disagreement is often due to inaccurate LMP recall, irregular cycles, or atypical ovulation timing.
Don't be alarmed by a small change in due date after early ultrasound — it's a routine part of obstetric care. Larger discrepancies, especially in the first trimester, can sometimes signal early growth concerns that warrant follow-up; this is a conversation to have with the provider.
Formula
- LMP = Last menstrual period (first day)
- due date (Naegele's rule) = LMP + 280 days (40 weeks)
Worked examples
LMP January 1
Weeks of gestation as of October 1: ~39 weeks. Estimated due date (Naegele): October 8 (LMP + 280 days). The actual delivery is statistically most likely in the 38–41 week window centered on October 8.
Adjusting for a long cycle
Woman with consistent 35-day cycle (vs. textbook 28-day). Ovulation typically occurs around day 21 instead of day 14 — a 7-day later conception. Naegele's rule gives a due date 7 days too early; many calculators allow the cycle length to be entered to correct this.
Early ultrasound disagreement
LMP-based dating: 11 weeks 3 days. Ultrasound at the same visit: crown-rump length consistent with 10 weeks 0 days — a 10-day difference. Per ACOG, ultrasound dating becomes the official date going forward, and the due date moves correspondingly later.
Frequently asked questions
Why is pregnancy dated from the last period rather than conception?
Because LMP is something most women can recall accurately, while conception is typically not datable to a specific day. Ovulation occurs about 14 days into a textbook 28-day cycle, so LMP dating runs about 2 weeks ahead of true gestational age. This is a convention, not a biological fact — every clinical timeline ("40 weeks pregnant") is in LMP-based time.
How accurate is the calculator's due date?
The Naegele's rule date is a statistical center, not a forecast. About 4% of pregnancies deliver on the date itself; about 88% deliver in the full-term window of 37–42 weeks. First-trimester ultrasound is the most accurate dating method and is the basis for the official due date when available.
What if my cycles aren't 28 days?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. Longer cycles (35-day) shift conception ~7 days later; shorter cycles (24-day) shift it ~4 days earlier. Many calculators let you enter cycle length to adjust. Early ultrasound dating supersedes any cycle-based correction in routine care.
When should I see a provider after a positive test?
Most U.S. guidelines suggest scheduling the first prenatal visit at 8–10 weeks of gestation. Earlier evaluation is reasonable for women with risk factors (prior miscarriage, diabetes, advanced maternal age, conditions managed by medications). At-home tests don't replace clinical confirmation.
What's the difference between embryonic and fetal age?
Embryonic age is time since fertilization. Fetal age (gestational age) is time since the first day of LMP. Gestational age is roughly 2 weeks longer. Almost all clinical communication uses gestational age.
Can the calculator detect a problem?
No — it's a dating tool only. It can't evaluate fetal viability, growth, or risk. If you have concerns (abdominal pain, bleeding, atypical symptoms, missed prenatal milestones), contact a healthcare provider directly. Calculators don't substitute for clinical evaluation.