liminfoPregnancy Calculator
Free web tool: Pregnancy Calculator
About Pregnancy Calculator
The Pregnancy Calculator estimates the most critical dates and milestones of a pregnancy from two input methods. The LMP (Last Menstrual Period) method is the standard obstetric approach: enter the first day of your last period, and the calculator adds 280 days (40 weeks) to find the estimated due date (EDD). It also computes the estimated conception date (LMP + 14 days, assuming ovulation on day 14 of a 28-day cycle), your current gestational age in weeks and days, and which trimester you are in.
For patients who conceived through in vitro fertilization (IVF), the calculator provides a dedicated egg retrieval / fertilization date method. Because the exact fertilization date is known in IVF, pregnancy duration is counted from that date rather than from the LMP. The calculator adds 266 days (38 weeks from fertilization, equivalent to 40 weeks from LMP) to the egg retrieval date to find the EDD.
Gestational age is calculated as the difference in days between today and the LMP date, expressed as weeks and remaining days. Trimester boundaries follow standard obstetric definitions: weeks 0–12 (1st trimester), weeks 13–27 (2nd trimester), and weeks 28+ (3rd trimester). All calculations run locally in the browser — no pregnancy data is sent to any server.
Key Features
- LMP method: due date (EDD) = LMP + 280 days using Naegele's rule
- IVF egg retrieval method: EDD = fertilization date + 266 days
- Real-time gestational age display in weeks and days from today
- Trimester identification (1st: 0–12w, 2nd: 13–27w, 3rd: 28w+)
- Estimated conception date (LMP + 14 days) for natural pregnancy
- Tab switch between LMP and IVF modes with no page reload
- 100% client-side — no personal health data sent to any server
- Dark mode and responsive layout for use on any device
Frequently Asked Questions
How is the due date calculated from the last menstrual period?
The due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. This formula was established in the early 19th century and remains the standard method used by obstetricians worldwide. It assumes an average 28-day cycle with ovulation on day 14.
Why is 280 days used instead of 266 days?
Pregnancy is counted from the first day of the last menstrual period, not from the actual day of fertilization. Since ovulation and fertilization typically occur about 14 days into the cycle, adding 280 days from LMP equals 266 days from fertilization. Both numbers represent the same biological duration; the LMP-based 280-day count is used because it is a known date, while the exact fertilization date is usually unknown in natural conception.
How does the IVF due date calculation differ?
In IVF, the egg retrieval and fertilization date is precisely known. Since there is no ambiguity about conception timing, the EDD is calculated as fertilization date + 266 days (38 weeks), bypassing the LMP assumption. This method is also applicable when a frozen embryo transfer date is known (adjust by adding 5 days for a day-5 blastocyst transfer, for example).
What are the trimester boundaries?
Standard obstetric trimesters are: 1st trimester = weeks 0 through 12; 2nd trimester = weeks 13 through 27; 3rd trimester = weeks 28 through 40+. This calculator uses these boundaries to display your current trimester based on your gestational age today.
How accurate is the estimated due date?
The EDD is an estimate, not a guarantee. Only about 5% of babies are born exactly on their due date. Normal delivery is considered to occur between 37 and 42 weeks gestation. The calculated EDD can shift if your cycle length differs significantly from 28 days; an ultrasound measurement in the first trimester provides a more precise estimate.
What is the estimated conception date?
The estimated conception date (LMP + 14 days) assumes ovulation occurred on day 14 of your cycle. In reality, ovulation timing varies by cycle length and individual factors. For women with longer cycles (e.g., 35 days), ovulation occurs around day 21, making the actual conception date about a week later than the estimate.
Can I use this calculator for a multiple pregnancy (twins, triplets)?
The EDD calculation is the same for singleton and multiple pregnancies from an obstetric dating standpoint. However, multiple pregnancies often deliver earlier (average 37 weeks for twins vs. 40 weeks for singletons). Consult your obstetrician for specific guidance on your pregnancy.
Is this calculator a substitute for prenatal care?
No. This tool provides general educational estimates for informational purposes only. Accurate dating, monitoring of fetal development, and medical advice must come from a licensed obstetrician or midwife. Always seek professional prenatal care for all pregnancy-related health decisions.