Gestational Age Calculator
Find out exactly how many weeks pregnant you are, your estimated due date, and which trimester you're in — from LMP, ultrasound, or IVF transfer date.
Calculate Your Gestational Age
What Is Gestational Age?
Accurate dating is one of the most important elements of prenatal care. Gestational age determines screening test timing, fetal growth assessment, and delivery planning. ACOG recommends that all pregnancies be dated using a combination of LMP and first-trimester ultrasound when available.
Medical Disclaimer: This tool provides general educational estimates. Always consult your prescribing physician or healthcare provider before making medication changes or interpreting results from population-based models.
Pregnancy Dating Methods Compared
There are three main ways to calculate gestational age. Each has different accuracy and appropriate use cases.
| Method | Accuracy | Best Used When | Limitations |
|---|---|---|---|
| LMP (Naegele's Rule) | ±2-3 weeks | Regular 28-day cycles, known LMP date | Assumes ovulation on day 14; inaccurate with irregular cycles |
| First Trimester Ultrasound | ±5-7 days | Before 14 weeks; gold standard for dating | Less accurate in 2nd/3rd trimester (±2-3 weeks) |
| IVF Transfer Date | ±1-2 days | IVF pregnancies (most precise method) | Only applicable to IVF pregnancies |
Trimester Guide
First Trimester (Weeks 1-12)
- Week 4: Implantation completes. Pregnancy test becomes positive.
- Week 6: Heartbeat detectable on ultrasound (transvaginal).
- Week 8: All major organs have begun to form. The embryo is now called a fetus.
- Week 10-12: First trimester screening (nuchal translucency, blood tests). Risk of miscarriage drops significantly after a heartbeat is confirmed at 8+ weeks.
Second Trimester (Weeks 13-26)
- Week 16: Sex may be visible on ultrasound. Many women begin to show.
- Week 18-20: Quickening — first fetal movements felt. Anatomy scan (20-week ultrasound) checks for structural abnormalities.
- Week 24: Viability threshold — survival outside the womb becomes possible with intensive care (~50-70% survival at specialized centers).
Third Trimester (Weeks 27-40)
- Week 28: Survival rate exceeds 90% if born prematurely. GBS screening and Rh immunoglobulin (if needed) are given.
- Week 32-34: Lungs are developing surfactant. Rapid brain growth. Fetus gains ~0.5 lb per week.
- Week 37: Early term. Previously considered "term," but ACOG now recommends waiting until 39 weeks for elective delivery.
- Week 39-40: Full term. Brain and lungs complete final maturation. Optimal window for delivery.
ACOG Term Definitions (2013)
In 2013, ACOG and SMFM replaced the single "term" designation with more precise categories to discourage elective early delivery:
| Classification | Gestational Age | Clinical Significance |
|---|---|---|
| Preterm | <37w0d | Increased risk of respiratory, neurological, and developmental complications |
| Early Term | 37w0d - 38w6d | Elective delivery not recommended unless medically indicated |
| Full Term | 39w0d - 40w6d | Optimal outcomes. Lowest risk of neonatal complications. |
| Late Term | 41w0d - 41w6d | Increased monitoring. Consider induction. |
| Post Term | ≥42w0d | Increased risk of stillbirth. Delivery recommended. |
Source: ACOG Committee Opinion No. 579 (2013), reaffirmed 2021.
Frequently Asked Questions
How many weeks pregnant am I?
Count the number of days from the first day of your last menstrual period (LMP) to today, then divide by 7. For example, if your LMP was 56 days ago, you are 8 weeks and 0 days pregnant. Use the calculator above for an exact result, including fetal age, due date, and trimester.
What is the difference between gestational age and fetal age?
Gestational age counts from the first day of your last period — about 2 weeks before conception actually occurred. Fetal age (conceptional age) counts from fertilization. So a pregnancy at 10 weeks gestational age has a fetus that is approximately 8 weeks old. Doctors use gestational age for all clinical decisions and scheduling.
How accurate is an LMP-based due date?
LMP-based dating assumes a 28-day cycle with ovulation on day 14 — which is true for only about 30% of women. If your cycles are longer or shorter, or irregular, LMP dating can be off by 2-3 weeks. First-trimester ultrasound (±5-7 days accuracy) is recommended to confirm or adjust the due date. ACOG states that if ultrasound dating differs from LMP by more than 7 days in the first trimester, the ultrasound date should be used.
What percentage of babies are born on their due date?
Only about 4-5% of babies are born on their exact due date. The due date is an estimate — most babies are born within a 5-week window (37-42 weeks). About 57% are born within 1 week of the due date. First-time mothers tend to deliver slightly later on average than experienced mothers.
Why does gestational age start before conception?
Historically, the last menstrual period was the only reliable date available before ultrasound technology existed. Since ovulation and conception dates were rarely known, counting from the LMP became the standard convention. This convention persists today because it provides a consistent framework that all healthcare providers use, even though it means "week 1" and "week 2" of pregnancy occur before fertilization.
Sources & Methodology
- American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. Committee Opinion No. 700. Obstet Gynecol. 2017;129(5):e150-e154.
- ACOG Committee Opinion No. 579. Definition of Term Pregnancy. Obstet Gynecol. 2013;122(5):1139-1140.
- Butt K, Lim K. Determination of Gestational Age by Ultrasound. J Obstet Gynaecol Can. 2014;36(2):171-181.
- Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol. 1996;174(1):278-281.
This calculator uses peer-reviewed formulas and clinical guidelines. Results are estimates and should not replace professional medical advice.