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Pregnancy Calculator
Calculate your due date and pregnancy week based on last menstrual period or conception date. Track trimester milestones. Free pregnancy due date calculator.
Pregnancy Calculator
Calculate your due date, track your pregnancy progress, and discover important milestones throughout your journey.
📅 Calculate Due Date
Most common method - uses Naegele's rule (LMP + 280 days)
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Estimated due date (EDD) is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). Pregnancy is divided into three trimesters: T1 (weeks 1–13, organogenesis, highest miscarriage risk), T2 (weeks 14–27, rapid fetal growth, anatomy scan at 18–22 weeks), T3 (weeks 28–40, lung maturation, positioning for birth). Gestational age is measured from LMP; fetal age is ~2 weeks less. Full-term: 39–40 weeks (ACOG); premature: <37 weeks; post-term: ≥42 weeks. First-trimester ultrasound (crown-rump length) is accurate to ±5–7 days.
Understanding Pregnancy & Due Date Calculation
Evidence-based obstetric science since 1756
280
Days average pregnancy
3
Trimesters
40
Weeks gestational
1756
Year Naegele's Rule established
🗓️ How Due Date Calculation Works
A due date, formally called the Estimated Date of Delivery (EDD) or Estimated Date of Confinement (EDC), marks the end of an average full-term pregnancy of 40 weeks (280 days) counted from the first day of the last menstrual period (LMP). The 40-week framework exists because most women ovulate around day 14 of their cycle, so the true fertilization window is approximately 2 weeks after the LMP begins.
In clinical practice, gestational age is measured from LMP even though the embryo is actually about 2 weeks younger than the gestational age suggests. This convention was standardized long before reliable conception-date testing was possible and remains universal today.
Naegele's Rule
The most widely used method for calculating due dates is Naegele's Rule, formulated by German obstetrician Franz Karl Naegele in 1756. The formula is elegantly simple:
Due Date = LMP + 280 days
Equivalently: LMP + 9 calendar months + 7 days
Example: LMP = January 1 → Due Date = October 8
LMP vs. Conception Date
🩸 LMP Method
- • Counts from the first day of last period
- • Assumes a regular 28-day cycle
- • Most commonly used in clinical settings
- • Ovulation assumed at day 14
- • Gestational age = fetal age + 2 weeks
- • Less precise for irregular cycles
🥚 Conception Date
- • Counts from known fertilization date
- • Adds 266 days (38 weeks)
- • More accurate when date is confirmed
- • Often used for IVF pregnancies
- • Fetal age = gestational age − 2 weeks
- • Requires reliable ovulation tracking
Clinical Note: First-trimester ultrasound (before 14 weeks) is considered the gold standard for dating accuracy, achieving a margin of error of ±3–5 days. After the first trimester, accuracy decreases and an LMP-based date is generally not revised.
📅 Pregnancy Timeline by Week
| Week | Trimester | Key Milestone | Baby Size Reference |
|---|---|---|---|
| W4 | 1st | Embryo implants in uterine wall; neural tube begins forming | Poppy seed (~2 mm) |
| W8 | 1st | Heart beating; limb buds present; all major organs forming | Raspberry (~1.6 cm) |
| W12 | 1st | Nuchal translucency scan; all organs formed; miscarriage risk drops sharply | Lime (~5.4 cm) |
| W16 | 2nd | Gender often visible on ultrasound; baby makes facial expressions | Avocado (~11.6 cm) |
| W20 | 2nd | Anatomy scan; halfway point; many mothers first feel kicks | Banana (~25 cm) |
| W24 | 2nd | Viability threshold; lungs developing; baby responds to sound | Corn cob (~30 cm) |
| W28 | 3rd | Eyes open; REM sleep cycles; glucose tolerance screening | Eggplant (~37 cm) |
| W32 | 3rd | Rapid weight gain; lungs nearly mature; baby may turn head-down | Butternut squash (~42 cm) |
| W36 | 3rd | Baby drops into pelvis (lightening); cervix may begin softening | Honeydew melon (~47 cm) |
| W40 | 3rd | Estimated due date; fully developed; average weight ~3.4 kg / 7.5 lb | Small pumpkin (~51 cm) |
🏥 Prenatal Care Schedule
| Week Range | Visit Frequency | Tests & Screenings |
|---|---|---|
| Wk 4–8 | Initial booking visit | Blood type, Rh factor, CBC, rubella immunity, STI screening, urine culture, genetic counseling referral |
| Wk 8–13 | Once | Nuchal translucency ultrasound; cell-free DNA / NIPT (optional); chorionic villus sampling if indicated |
| Wk 14–27 | Every 4 weeks | Anatomy scan at 18–22 weeks; AFP/quad screen at 15–20 weeks; amniocentesis if indicated |
| Wk 28–35 | Every 2 weeks | Glucose challenge test at 24–28 weeks; Tdap vaccine at 27–36 weeks; RhoGAM for Rh-negative patients |
| Wk 36–40 | Weekly | Group B Strep (GBS) culture at 35–37 weeks; fetal position check; cervical exam; biophysical profile if post-term |
⚖️ Pregnancy Weight Gain Guidelines
Based on the 2009 Institute of Medicine (IOM) guidelines, adopted by ACOG.
| Pre-pregnancy BMI | Category | Total Recommended Gain | Weekly Gain (2nd–3rd Trimester) |
|---|---|---|---|
| < 18.5 | Underweight | 12.5–18 kg (28–40 lb) | ~0.5 kg / ~1 lb |
| 18.5–24.9 | Normal weight | 11.5–16 kg (25–35 lb) | ~0.4 kg / ~1 lb |
| 25.0–29.9 | Overweight | 7–11.5 kg (15–25 lb) | ~0.3 kg / ~0.6 lb |
| ≥ 30.0 | Obese | 5–9 kg (11–20 lb) | ~0.2 kg / ~0.5 lb |
| Any (twins) | Twin pregnancy | 17–25 kg (37–54 lb) | ~0.7 kg / ~1.5 lb |
* Consult your healthcare provider for personalized recommendations.
📜 History of Pregnancy Dating
1756
Naegele's Rule formulated
Franz Karl Naegele publishes his due-date calculation method (LMP + 280 days), establishing the framework still used worldwide today.
1827
Egg cell discovered
Karl Ernst von Baer identifies the human ovum, revolutionizing understanding of fertilization and conception timing in mammals.
1950s
Obstetric ultrasound begins
Ian Donald pioneers obstetric ultrasound in Glasgow, offering real-time fetal imaging and a new, more accurate route to dating gestational age.
1978
First IVF birth
Louise Brown is born via in vitro fertilization, bringing precise date-of-conception knowledge and new due-date calculation methods for assisted reproduction.
1990s
Prenatal genetic testing advances
First-trimester combined screening and amniocentesis become routine, enabling chromosomal analysis alongside gestational age confirmation.
2000s
3D/4D ultrasound era
Three-dimensional and real-time 4D ultrasound imaging bring detailed anatomical views, improving fetal growth assessment and anomaly detection.
🎯 How Accurate Is Your Due Date?
LMP Method
Assumes a perfectly regular 28-day cycle with ovulation on day 14. Studies show accuracy within ±2 weeks in most regular-cycle pregnancies.
Margin: ±10–14 days
1st-Trimester Ultrasound
Crown-rump length measurement before 14 weeks is the most precise dating tool available in clinical practice.
Margin: ±3–5 days
2nd/3rd Trimester Ultrasound
Later scans assess head circumference, femur length, and abdominal circumference. Normal biological variation increases the error window.
Margin: ±2–3 weeks
Why Babies Don't Arrive on the Due Date
Only about 4–5% of babies are born exactly on their estimated due date. Natural labor is triggered by a complex cascade of hormonal signals from both baby and mother, with biological variability spanning several weeks. A term birth is defined as 37–42 weeks gestation. Babies born between 37 and 41 weeks and 6 days are considered "term" without medical concern about gestational age alone.
🔬 Research & Clinical Guidelines
ACOG Practice Bulletin
Methods for Estimating the Due Date
ACOG recommends ultrasound in the first trimester as the gold standard for gestational age assignment when discordant by more than 5 days from LMP-based estimates.
ACOG Committee Opinion No. 700 →British Medical Journal
Distribution of Gestational Length at Birth
Multinational BMJ research shows spontaneous births cluster between 38 and 42 weeks, with the median at 39.5 weeks, reinforcing that 40-week due dates are estimates, not targets.
Morken et al., BMJ Open, 2014
WHO Guidelines
WHO Antenatal Care Recommendations
The World Health Organization recommends a minimum of 8 antenatal care contacts for a positive pregnancy experience, with early booking before 12 weeks whenever possible.
WHO ANC Model 2016 →💡 Pregnancy Myths vs. Facts
Myth
Your due date is an exact prediction of when your baby will be born.
Fact
Only 4–5% of babies are born on their due date. Most arrive within 2 weeks before or after. The due date is the midpoint of a normal distribution of labor onset, not a deadline.
Myth
Morning sickness only happens in the morning.
Fact
Despite the name, nausea and vomiting can happen at any time of day or night. About 80% of pregnant women experience nausea, and for many it is persistent throughout the day, especially in the first trimester.
Myth
You need to "eat for two" during pregnancy.
Fact
You only need an extra ~300 calories per day in the second and third trimesters — roughly equivalent to a healthy snack, not a full additional meal. Excessive caloric intake increases gestational weight gain risks.
Myth
Heartburn during pregnancy means your baby will have lots of hair.
Fact
While one small Johns Hopkins study found a weak correlation, heartburn is overwhelmingly caused by progesterone relaxing the lower esophageal sphincter and the growing uterus putting upward pressure on the stomach.
Myth
Exercise is dangerous during pregnancy.
Fact
For low-risk pregnancies, ACOG actively recommends 150 minutes of moderate-intensity aerobic activity per week. Exercise reduces risk of gestational diabetes, preeclampsia, excessive weight gain, and cesarean delivery.
Myth
If the fetal heartbeat is fast, it's a girl; slow means a boy.
Fact
No scientific evidence supports fetal heart rate as a reliable sex predictor. The average fetal heart rate is 120–160 bpm in both sexes, varying with gestational age and activity level rather than sex.
❓ Frequently Asked Questions
How accurate is my estimated due date?▼
What is the difference between gestational age and fetal age?▼
How is the due date calculated for IVF pregnancies?▼
What counts as early, late, or post-term birth?▼
When can I feel my baby move for the first time?▼
What is the Group B Strep (GBS) test and when is it done?▼
Can my due date change after it is first set?▼
What is a biophysical profile (BPP) and when is it ordered?▼
What is the difference between a miscarriage and a stillbirth?▼
How is gestational age determined if I do not know my LMP?▼
What prenatal vitamins and nutrients are most important?▼
What is preeclampsia and how is it monitored?▼
📋 References & Further Reading
- • ACOG Committee Opinion No. 700: Methods for Estimating the Due Date (2017). American College of Obstetricians and Gynecologists.
- • Naegele, F.K. (1756). Erfahrungen und Abhandlungen aus dem Gebiethe der Krankheiten des weiblichen Geschlechts. Mannheim.
- • Morken N-H, et al. Distribution of gestational length in a multi-country cohort. BJOG. 2014;121(7):850–857.
- • World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience. Geneva: WHO; 2016.
- • Institute of Medicine. Weight Gain During Pregnancy: Reexamining the Guidelines. Washington, DC: National Academies Press; 2009.
- • ACOG Committee Opinion 804: Physical Activity and Exercise During Pregnancy and the Postpartum Period. 2020.
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Pregnancy Calculator — Complete Guide
Due date calculation methods, pregnancy weeks by trimester, Naegele's rule, fetal development milestones, and prenatal care timeline.
40 weeks
Average pregnancy duration
280 days
From LMP to due date
3
Trimesters of pregnancy
±2 weeks
Normal delivery window
How Is a Due Date Calculated?
A pregnancy due date (estimated date of delivery, EDD) marks 40 weeks — 280 days — from the first day of the last menstrual period (LMP). This convention is used because ovulation and conception typically occur around day 14 of a 28-day cycle, making LMP a consistent and verifiable starting point even when the exact conception date is unknown.
The standard formula is Naegele's Rule: subtract 3 months from the LMP date, add 7 days, and add 1 year. Only approximately 4% of babies are born on their exact due date — the normal range is 37–42 weeks (term pregnancy). Babies born before 37 weeks are preterm; after 42 weeks is post-term.
Ultrasound dating, particularly the first-trimester crown-rump length (CRL) measurement at 7–13 weeks, is considered more accurate than LMP-based dates — especially for women with irregular cycles. If ultrasound and LMP dates differ by more than 7 days (first trimester), the ultrasound date is typically used to set the EDD.
Due Date Calculation Methods
EDD = LMP + 280 days
or equivalently:
EDD = LMP − 3 months + 7 days + 1 year
Example (LMP = 1 March 2025):
Step 1: 1 March − 3 months = 1 December 2024
Step 2: 1 December + 7 days = 8 December 2024
Step 3: 8 December 2024 + 1 year = 8 December 2025
EDD = 8 December 2025
Adjust for cycle length:
Standard assumes 28-day cycle.
If cycle = 35 days:
Add (35 − 28) = 7 extra days
If cycle = 21 days:
Subtract (28 − 21) = 7 days
Accuracy: ±2 weeks for most womenNaegele's rule was proposed by Franz Karl Naegele in 1812 and remains the standard clinical calculation. It assumes a 28-day cycle with ovulation on day 14. It can be adjusted for cycle length variations.
If conception date is known: EDD = Conception date + 266 days (38 weeks from conception) Note: 280 days from LMP = 266 days from ovulation/conception (2 weeks later) Example: Conception: 14 March 2025 EDD = 14 March + 266 days EDD = 5 December 2025 When conception is known: • IVF transfers (exact date known) • Confirmed ovulation with testing • Single unprotected encounter IVF transfer calculation: Day 3 transfer: add 263 days Day 5 transfer: add 261 days
For IVF pregnancies, the transfer date is precisely known. Adjustments are made based on embryo age at transfer: a day-5 blastocyst transfer subtracts 5 days from the 266-day conception formula.
Pregnancy Trimesters — Week by Week
First Trimester
Weeks 1–12
- •Week 4: Positive pregnancy test possible
- •Week 5–6: Heartbeat detectable by transvaginal US
- •Week 8: Embryo becomes a fetus
- •Week 10: Basic organ systems formed
- •Week 12: Risk of miscarriage drops significantly
Most critical period for organ formation. Folic acid 400–800 mcg/day essential to prevent neural tube defects.
Second Trimester
Weeks 13–26
- •Week 13–16: Gender may be visible on ultrasound
- •Week 18–22: Anatomy scan (detailed ultrasound)
- •Week 20: Fundal height = 20 cm (navel level)
- •Week 24: Viability threshold (with intensive NICU care)
- •Week 25–26: Lungs begin producing surfactant
Often called the 'honeymoon trimester' — morning sickness usually resolves and energy returns. Fetal movements typically felt (quickening) at 18–22 weeks.
Third Trimester
Weeks 27–40
- •Week 28: Fetus is 36 cm, weighs ~1 kg
- •Week 32: Most body systems functional
- •Week 36: Considered 'late preterm'
- •Week 37: Full term begins
- •Week 40: Expected due date
- •Week 41–42: Post-dates monitoring begins
Baby gains most of its weight during this trimester. GBS screening at 36 weeks. Prenatal visits increase to weekly after 36 weeks.
Prenatal Visit Schedule
| Weeks | Visit Frequency | Key Tests/Milestones |
|---|---|---|
| 4–8 | First visit | Confirm pregnancy, blood type, CBC, STI screening, genetic carrier screening |
| 10–13 | Every 4 weeks | NIPT/cell-free DNA testing (optional), first trimester screening |
| 18–22 | Once | Anatomy ultrasound — checks all fetal organs |
| 24–28 | Every 4 weeks | Glucose tolerance test (gestational diabetes), Rh factor check |
| 28–36 | Every 2 weeks | Blood pressure, fundal height, fetal position |
| 36+ | Weekly | GBS culture (36 wk), fetal monitoring if post-dates |
Frequently Asked Questions
How accurate is a due date calculator?▼
Due date calculators based on LMP are accurate to within ±2 weeks for most women with regular cycles. The actual variation in gestational length is significant — only about 4% of babies are born on their exact due date. First-trimester ultrasound dating is more accurate and is used to confirm or revise the EDD.
What does gestational age vs. fetal age mean?▼
Gestational age is counted from the LMP (the clinical standard) and is typically 2 weeks longer than fetal age (counted from conception). When a doctor says 'you are 8 weeks pregnant,' they mean 8 weeks from LMP — your baby is actually about 6 weeks post-conception. All due date calculators use gestational age.
When should I take a pregnancy test?▼
Home urine pregnancy tests detect hCG from about 10–14 days after conception — typically around the time of a missed period. Testing too early may give false negatives. For most reliable results, test the first morning urine on the day of your expected period or after.
References & Clinical Sources
- ACOG Practice Bulletin. Methods for Estimating the Due Date. Obstetrics & Gynecology. 2017;129(5):e150–e154.
- Naegele FC. Lehrbuch der Geburtshülfe. Heidelberg: Mohr und Zimmer, 1812.
- WHO. WHO recommendations on antenatal care for a positive pregnancy experience. 2016.
- Mongelli M, et al. Influence of conception date on the accuracy of gestational age estimation. Ultrasound Obstet Gynecol. 1996;8(5):318–22.