# Calculating Pregnancy Due Date

MyMonthlyCycles has several pregnancy calculators that can forecast your estimated due date and/or conception date. Below we explain how these calculators do the date math. When using these calculators, always remember that all calculated dates are estimates - they are not exact. Only about 5% of babies (1 in 20) are born on their estimated due dates. For most women (95%), there will always be a margin of error of a few weeks on either side of the estimated due date. If you are pregnant, you should always talk to your own medical doctor, including about other ways your doctor may use to determine your due date.

Our due date calculators allow you flexibility in the criteria you can put in -- a great way to what-if calculate. This also means that you can put in date criteria that is outside the range of your cycles. For example, you might specify a past period date that did not exist; or one that is so far into the future that such a period may not occur due to your cycle variations up to that point. So, when what-if calculating, particularly well into the future, please keep in mind that estimated dates may not be feasible at all.

Whether calculating based on your actual data, or what-if-ing, the date calculations are the same. Without further ado...

## Pregnancy Calculator - Based on Last Menstrual Period

Your estimated due date (also referred to as EDD, or estimated delivery date) is based on Naegele's Rule*, where a pregnancy is considered to last 280 days (40 weeks) from the date of your last menstrual period (LMP). This version of our calculator takes your LMP and adds 280 days, to yield your estimated due date.

Note that under Naegele's Rule, you are not pregnant for the full 40 weeks. Rather, your gestation period is considered to be 266 days (38 weeks) from the date of conception, which is assumed to be on day 14 of a 28 day cycle. Of course, many women do not have 28 day cycles, and do not ovulate on day 14 of their cycle.

Pregnancy Calcuator - Based on LMP

## Pregnancy Calculator - Based on Approximate Conception Date

Your estimated due date (also referred to as EDD, or estimated delivery date) is based on Naegele's Rule*, where gestation is approximately 266 days (38 weeks) from the date conception.

This calculator makes two calculations. It first calculates your approximate conception date based on your last menstrual period (LMP), the average number of days in your cycle, and your luteal phase. It then calculates your EDD by adding 266 days to your conception date.

Date of Conception Calculation

(LMP date + Average Cycle Length in Days) - Luteal Phase in Days = Approximate Date of Ovulation

Under Naegele's Rule, your ovulation date is also considered to be the date you conceived. Further, Naegele's rule assumes a 28 day cycle and a 14 day luteal phase (meaning that ovulation would occur on day 14 of your cycle ((LMP + 28 days)-14 days). Our calculator allows you to refine the ovulation/conception calculation based on your unique cycle history. If you know your average cycle length and/or luteal phase, you can provide this information.

For your convenience, if you are a logged in member, your average cycle length is automatically calculated and displayed as your default (which you can change), if at least 1 complete cycle (two consecutive menstrual periods) has been tracked.

Pregnancy Calculator - Based on Approximate Conception Date

## Pregnancy Calculator - Based on Specified Conception Date

Estimated due date is calculated based on a gestation period of 266 days (38 weeks).

If you have a good idea when you conceived, or if you just want to what-if (if I were to conceive on {date} or if I did conceive on {date}), this is the calculator you'll want to use.

Pregnancy Calculator - Based on Specified Conception Date

## Reverse Pregnancy Calculator

This what-if calculator approximates when you'd need to conceive if you were to deliver on the date you specify. Approximate conception date is calculated by subtracting 266 days (38 weeks) from the due date you supply on the criteria screen.

Reverse Pregnancy Calculator

## Luteal Phase

The luteal phase, often referred to as DPO (days past ovulation), is the third phase of the menstrual cycle, and is the time span occurring after ovulation until the day before your next menstrual period begins. For many women with regular cycles, the luteal phase is generally from 10 to 16 days, with 14 days being the average length.

When using the Pregnancy Calculator - Based on Approximate Conception Date, there is an entry field for luteal phase. If you know your luteal phase, for instance because you have charted your fertility cycles in the past or have used ovulation prediction tests, you can override the default by selecting your luteal length from the drop-down box. If you do not know your luteal length, you should leave the default of 14 days. Many pregnancy calculators automatically use 14 days.

This is an advanced feature and should be used by those who have already used other means to determine their luteal length. It enables you to fine-tune the calculation based on what you've previously learned about your cycle.

Note that the valid luteal length range for this calculator is between 10 and 16 days. The calculator cannot be used with luteal lengths outside this range.

*Dr. Franz Carl Naegele was a German obstetrician, practicing in the mid 1800s. Many doctors and caregivers use Naegele's Rule today when approximating EDD.

Disclaimer: All information provided on the pregnancy calculator, including predicted dates of conception and baby's due date, is for informational purposes only. All calculated dates are estimates and should be considered as such: since each pregnancy is unique, there are no guarantees that the dates are or will be accurate for you. If you are pregnant, or believe you are pregnant, you should always contact your own medical doctor with any questions you may have.