Pregnancy Conception Calculator — Free Gestational and Fetal Age Tool
Estimate when you conceived and track both gestational age and fetal age simultaneously. See your conception date, fertile window, trimester, and due date based on your last menstrual period.
Conception Timeline
When Did You Conceive?
Estimated Conception Date
Monday, December 1, 2025
Gestational Age
14w 2d
Fetal Age
12w 2d
Summary: Based on your LMP of Monday, November 17, 2025 with a 28-day cycle, you likely conceived around Monday, December 1, 2025. Your gestational age is 14 weeks and 2 days, while your baby's actual fetal age is approximately 12 weeks and 2 days. The 2-week difference exists because gestational age counts from your LMP, which is about 2 weeks before ovulation and conception.
How to Use the Pregnancy Conception Calculator
- Enter the first day of your last menstrual period (LMP): Use the date picker to select the date your most recent period started. This is the foundation for all calculations. The calculator allows dates within the past 300 days to cover the full range of pregnancy. If you are uncertain of the exact date, use your best estimate and discuss with your healthcare provider for confirmation.
- Adjust your average cycle length: Enter your typical menstrual cycle length. The default is 28 days. If your cycles consistently run shorter or longer, adjusting this value significantly improves the accuracy of both the conception date and fetal age estimates. The calculator accepts values from 20 to 45 days. For irregular cycles, average your last three to six cycles for the best estimate.
- Review your complete pregnancy timeline: The results instantly show your estimated conception date, your current gestational age (from LMP) and fetal age (from conception), the fertile window during which conception likely occurred, your current trimester, the number of days until your due date, and the estimated due date. The gestational and fetal ages are shown side by side to help you understand both dating systems.
This calculator shows both gestational age (used by healthcare providers) and fetal age (actual developmental time) to give you a complete picture. The 2-week difference between these measures is normal and expected. Always use gestational age when communicating with your healthcare team, as all medical guidelines and prenatal schedules reference gestational age.
Conception and Age Calculation Formulas
Estimated Conception Date:
Conception = LMP + (Cycle Length - 14) days Gestational Age:
Gestational Age = Today - LMP (in weeks and days) Fetal Age:
Fetal Age = Gestational Age - (Cycle Length - 14) days Due Date:
Due Date = LMP + 280 + (Cycle Length - 28) days Variables Explained
- Cycle Length - 14: Calculates the estimated day of ovulation. The luteal phase (post-ovulation) is relatively constant at about 14 days, so subtracting 14 from the total cycle length gives the ovulation day. This determines the conception date and the fetal age offset.
- Gestational Age: The standard pregnancy dating system used worldwide. Measured from LMP, it includes the approximately 2 weeks before conception. All medical references use this system.
- Fetal Age: The actual developmental age of the embryo or fetus. Starts from conception and is always less than gestational age by about 2 weeks (varies with cycle length). This reflects how long your baby has truly been developing.
- Trimester Assignment: First trimester = gestational weeks 1 to 12. Second trimester = weeks 13 to 27. Third trimester = weeks 28 to 40. These divisions align with major developmental transitions and prenatal care phases.
Step-by-Step Example
Calculate for an LMP of October 1, 2025, with a 32-day cycle, evaluated on February 16, 2026:
- Ovulation day: 32 - 14 = day 18
- Conception date: October 1 + 18 = October 19, 2025
- Days since LMP: 138 days
- Gestational age: 138 / 7 = 19 weeks, 5 days
- Fetal age: (138 - 18) / 7 = 120 / 7 = 17 weeks, 1 day
- Due date: October 1 + 284 = July 12, 2026
- Days until due: 146
- Trimester: 2nd
Practical Examples
Example 1: Taylor Tracking Development
Taylor, 29, has regular 28-day cycles. Her LMP was September 15, 2025. She wants to understand the difference between the gestational age her doctor mentions and her baby's actual developmental age (as of February 16, 2026).
- Conception date: September 15 + 14 = September 29, 2025
- Gestational age: 154 days = 22 weeks, 0 days
- Fetal age: 140 days = 20 weeks, 0 days
- Due date: September 15 + 280 = June 22, 2026
- Trimester: 2nd
Taylor now understands that when her doctor says "22 weeks," her baby has actually been developing for about 20 weeks since fertilization. This distinction helps her interpret prenatal resources that reference developmental milestones, as some resources use gestational age while others use fetal age. Her anatomy scan at 20 gestational weeks confirmed normal development consistent with this timeline.
Example 2: Ashley with a Short Cycle
Ashley, 26, has 24-day cycles. Her LMP was August 10, 2025. She wants to see how her shorter cycle affects the conception estimate and fetal age.
- Ovulation day: 24 - 14 = day 10
- Conception date: August 10 + 10 = August 20, 2025
- Gestational age: 190 days = 27 weeks, 1 day
- Fetal age: 180 days = 25 weeks, 5 days
- Due date: August 10 + 276 = May 13, 2026
- Trimester: 2nd (approaching 3rd at week 28)
With Ashley's shorter cycle, the gap between gestational and fetal age is only 10 days instead of the standard 14. This means her baby has been developing for slightly longer than what a standard 28-day cycle calculation would suggest. Her early ultrasound confirmed the adjusted dating was more accurate than the unadjusted estimate.
Example 3: Rebecca Planning Medical Leave
Rebecca, 35, is a nurse with 30-day cycles. Her LMP was October 20, 2025. She needs to coordinate medical leave with her hospital and understand exact pregnancy timing.
- Conception date: October 20 + 16 = November 5, 2025
- Fertile window: October 31 to November 6, 2025
- Gestational age on Feb 16, 2026: 119 days = 17 weeks, 0 days
- Fetal age: 103 days = 14 weeks, 5 days
- Due date: October 20 + 282 = July 29, 2026
Rebecca uses the due date to begin her leave planning. She schedules project handoffs starting in late June and arranges coverage for July shifts. Understanding both age measurements helps her communicate clearly with her own OB-GYN while also discussing her timeline with hospital administration. She references the pregnancy weight gain calculator to track her recommended weight gain through each trimester.
Gestational Age vs Fetal Age Reference Table
| Gestational Age | Fetal Age (28-day cycle) | Trimester | Key Development |
|---|---|---|---|
| 4 weeks | 2 weeks | 1st | Implantation complete, cells specializing |
| 8 weeks | 6 weeks | 1st | Heart beating, major organs forming |
| 12 weeks | 10 weeks | 1st | Reflexes begin, organs functional |
| 16 weeks | 14 weeks | 2nd | Movement begins, features recognizable |
| 20 weeks | 18 weeks | 2nd | Halfway point, anatomy scan |
| 28 weeks | 26 weeks | 3rd | Eyes open, sleep-wake cycles |
| 36 weeks | 34 weeks | 3rd | Lungs maturing, gaining weight rapidly |
| 40 weeks | 38 weeks | 3rd | Full term, ready for birth |
Tips and Complete Guide to Pregnancy Age Tracking
Understanding the Two Age Systems
The dual age system in pregnancy can be confusing, but each serves a purpose. Gestational age provides a universally consistent reference that all healthcare providers use, enabling standardized prenatal care schedules and screening timelines. Fetal age gives a more intuitive understanding of actual development — when a resource says "at 6 weeks of development, the heart begins beating," it is referring to 6 weeks of fetal age (8 weeks gestational). Understanding both systems helps you navigate pregnancy information more effectively and communicate clearly with your healthcare team.
How Conception Timing Affects Early Pregnancy Dating
In the earliest weeks, the difference between gestational and fetal age has practical implications. A positive home pregnancy test usually appears around 4 weeks gestational age (2 weeks after conception), when hCG levels become detectable. Morning sickness typically starts around 6 weeks gestational (4 weeks fetal). The first heartbeat is usually detected around 6 to 7 weeks gestational (4 to 5 weeks fetal). An early ultrasound at 7 weeks gestational is measuring an embryo that has actually been developing for only about 5 weeks. Knowing both ages helps set realistic expectations for early milestones and test timing.
When Conception Dating Matters Most
Accurate conception dating is most important for timing prenatal screenings correctly. First-trimester screening (nuchal translucency measurement and blood work) must be performed between 11 and 13 weeks gestational age. The anatomy scan is optimally performed at 18 to 22 weeks gestational. Glucose tolerance testing for gestational diabetes occurs at 24 to 28 weeks gestational. If the conception date is miscalculated, these screenings could be performed too early or too late, potentially affecting accuracy. Our due date calculator provides additional tools for confirming your pregnancy timeline.
Tracking Development with Fetal Age
Fetal age provides a clearer lens for understanding developmental milestones. At 3 weeks fetal age (5 gestational), the neural tube is forming. At 6 weeks fetal (8 gestational), all major organs have begun development. At 12 weeks fetal (14 gestational), the fetus can make sucking motions and the sex organs are differentiating. At 20 weeks fetal (22 gestational), the fetus responds to sound. At 28 weeks fetal (30 gestational), the brain is developing rapid connections. Tracking both ages simultaneously gives you the complete picture of your pregnancy journey.
Common Mistakes to Avoid
- Confusing gestational and fetal age in conversations: Always use gestational age with healthcare providers and in medical contexts. Save fetal age for personal understanding and developmental tracking.
- Not adjusting for cycle length: A 35-day cycle means ovulation was around day 21, not day 14. The fetal age offset changes from 14 days to 21 days, which can matter for early screening timing.
- Assuming all pregnancy resources use the same age system: Medical guidelines, apps, and informational websites vary in whether they reference gestational or fetal age. Check which system is being used before comparing milestones.
- Using this calculator for critical medical decisions: This tool provides estimates for personal understanding. Always rely on your healthcare provider and ultrasound dating for medical decisions about screening timing, intervention, and delivery planning.
Frequently Asked Questions
Gestational age counts from the first day of the last menstrual period (LMP), which is typically about 2 weeks before conception. Fetal age (also called embryonic age) counts from the actual date of fertilization. The difference between the two is approximately 14 days for a standard 28-day cycle, but it varies with cycle length. For a 35-day cycle, the difference would be about 21 days. Healthcare providers use gestational age as the standard for all pregnancy milestones, screening schedules, and due date calculations because the LMP is a known date, while the exact moment of fertilization is rarely certain. Our calculator shows both ages simultaneously so you can understand where your baby stands developmentally.
This pregnancy conception calculator focuses on providing a comprehensive pregnancy timeline centered around the conception date. While a standard conception calculator simply estimates when conception occurred, this tool additionally shows the fetal age (how long your baby has actually been developing), the gestational age, the trimester you are in, and the countdown to your due date. It emphasizes the distinction between gestational and fetal age, giving you a clearer picture of your baby's actual developmental stage. This is particularly useful for tracking developmental milestones, understanding when certain prenatal tests are appropriate, and communicating with your healthcare provider.
The 2-week difference (approximately) exists because gestational age starts counting from the first day of your last menstrual period, which is about 2 weeks before ovulation and conception. In a standard 28-day cycle, ovulation occurs around day 14, so the first 2 weeks of gestational age are actually the pre-conception phase when the egg is maturing and preparing for release. This convention was established long before ultrasound technology because the LMP was the only reliably known date. For individuals with cycles longer or shorter than 28 days, the difference between gestational and fetal age may be more or less than exactly 2 weeks.
No calculator can determine the exact day of conception with absolute certainty. The calculator provides an estimate based on the average timing of ovulation within your menstrual cycle. Actual conception timing can vary because sperm survive in the reproductive tract for up to 5 days, the egg is viable for 12 to 24 hours after ovulation, and ovulation may not occur exactly on the predicted day even in regular cycles. The estimate is most reliable for individuals with consistent, regular cycles. For the most accurate dating, a first-trimester ultrasound measuring crown-rump length can confirm gestational age to within 3 to 5 days.
Cycle length changes the estimated conception date, which in turn affects fetal age. In a 28-day cycle, ovulation occurs around day 14, meaning the fetal age is approximately gestational age minus 14 days. In a 35-day cycle, ovulation occurs around day 21, so fetal age is gestational age minus 21 days. In a 24-day cycle, ovulation occurs around day 10, and fetal age is gestational age minus 10 days. This is why entering your actual cycle length matters — it changes when we estimate conception happened and therefore how long your baby has actually been developing. Without cycle adjustment, fetal age estimates for longer or shorter cycles would be off by up to a week.
Developmental milestones are typically referenced by gestational age in medical literature, but understanding fetal age gives additional context. At 8 weeks gestational (6 weeks fetal), all major organs have begun forming and the heart is beating. At 12 weeks gestational (10 weeks fetal), the fetus has recognizable human features and reflexes begin. At 20 weeks gestational (18 weeks fetal), the halfway point, most organs are functional and movements are felt. At 24 weeks gestational (22 weeks fetal), the viability threshold is reached. At 37 weeks gestational (35 weeks fetal), the baby is considered early term. Knowing fetal age helps appreciate that your baby has been developing for less time than the gestational week count suggests.
Always use gestational age when communicating with your healthcare provider unless they specifically ask for fetal age. The entire medical system — including screening schedules, due date calculations, growth charts, and developmental milestones — is built around gestational age. When your provider says you are 20 weeks pregnant, they mean 20 weeks from your LMP, not 20 weeks since conception. Using fetal age in medical conversations could cause confusion and potentially affect the timing of important tests or interventions. This calculator shows fetal age for your personal understanding of your baby's developmental timeline, but gestational age is the language of prenatal care.
The fertile window shown is based on the estimated ovulation date, which is calculated as the cycle length minus 14 days after LMP. For individuals with regular, predictable cycles, this estimate is generally accurate to within 1 to 2 days. The 6-day window (5 days before ovulation plus ovulation day) accounts for sperm survival time and egg viability. However, for irregular cycles, the actual fertile window may shift. Factors like stress, illness, travel, and hormonal changes can cause ovulation to occur earlier or later than predicted. For more precise ovulation tracking, consider using basal body temperature, cervical mucus observation, or ovulation predictor kits alongside this calculator.
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Disclaimer: This calculator is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional for medical guidance.
Last updated: February 23, 2026
Sources
- American College of Obstetricians and Gynecologists (ACOG) — Methods for Estimating the Due Date: acog.org/clinical/clinical-guidance
- ACOG — How Your Fetus Grows During Pregnancy: acog.org/womens-health/faqs
- National Institute of Child Health and Human Development — Pregnancy Information: nichd.nih.gov/health/topics/pregnancy