Pregnancy Calculator
Discover your estimated due date, track your current gestational week, and unlock major pregnancy milestones based on your LMP, conception date, or IVF transfer.
Understanding Your Pregnancy Timeline
Pregnancy math is confusing. You are technically considered two weeks pregnant before conception even occurs.
The standard human pregnancy lasts approximately 280 days (40 weeks) from the first day of your last menstrual period (LMP). While this seems confusing, it is the medical standard because the exact day of ovulation and fertilization is usually impossible to pinpoint for women conceiving naturally.
Calculation Methods Explained
- Naegele’s Rule (LMP Method) This is the most common formula. It takes the date of your last period, adds 7 days, and subtracts 3 months. Our calculator adjusts this rule if your average cycle length is longer or shorter than 28 days, ensuring a far more accurate EDD.
- Conception Date Method If you were tracking ovulation via basal body temperature or strips, you might know your exact conception date. In this case, we simply add exactly 266 days (38 weeks) to that date to find your due date.
- IVF Transfer Method In Vitro Fertilization requires exact math. We calculate 266 days from your embryo transfer date, and then subtract the age of the embryo (usually 3 days or 5 days) that was transferred.
The Ultrasound Exception
While mathematical formulas are a great starting point, a first-trimester dating ultrasound is the most medically accurate way to determine a due date. During early pregnancy, all fetuses grow at a very consistent rate.
If your doctor’s ultrasound dates you more than 5 to 7 days differently than your LMP date, they will likely officially change your EDD to match the ultrasound measurements.
Pregnancy Timeline FAQs
10 common questions about due dates and trimesters.
The most common method (Naegele’s rule) calculates your Estimated Due Date (EDD) by taking the first day of your Last Menstrual Period (LMP), adding 7 days, and then adding 9 months. This assumes a standard 28-day cycle.
Because it’s very difficult to know the exact day of ovulation and conception for most women. However, the start of a menstrual period is a definitive, observable date. This is why you are considered ‘two weeks pregnant’ on the day you actually conceive.
If your cycle is longer than 28 days, your due date will be pushed back. If it’s shorter, your due date will be brought forward. Our calculator allows you to input your specific cycle length to adjust the EDD accurately.
An early ultrasound (performed in the first trimester between 8 and 13 weeks) is considered the most accurate method to date a pregnancy. If your ultrasound EDD differs from your LMP EDD by more than 5-7 days, your doctor will usually change your official due date to match the ultrasound.
IVF due dates are incredibly precise because the exact date of fertilization is known. It is calculated by adding 266 days to the transfer date and then subtracting the age of the embryo (e.g., subtracting 3 days for a Day-3 transfer, or 5 days for a Day-5 transfer).
Only about 4% to 5% of babies are born on their exact estimated due date. The EDD is just an estimate; a pregnancy is considered ‘full term’ anywhere between 39 weeks and 40 weeks and 6 days.
The second trimester officially begins at week 14 of your pregnancy (13 weeks and 0 days to 13 weeks and 6 days is the end of the first trimester).
While doctors rarely change an official due date based on third-trimester ultrasounds (because babies grow at vastly different rates late in pregnancy), your actual delivery date may change if medical interventions like inductions or C-sections become necessary.
A baby born before 37 weeks of gestation is considered premature. Babies born between 37 and 38 weeks are considered ‘early term’.
Pregnancy is tracked in weeks rather than months because months vary in length. A 40-week pregnancy is roughly 9 months and 1 week long. Generally, you divide your weeks by 4.3 to get a rough estimate of months.
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