Due Date Calculator

So you’ve gotten the BFP (big fat positive) and now you’re wondering, “when is my due date?” Enter your info in our due date calculator to find out!

When was your last period?

Please choose the FIRST day your period started.

Average cycle: days


What day did you conceive?

Conception is approximately two days after intercourse.

Due Date Calculator: How do you calculate due date?

So you’ve gotten the BFP (big fat positive) and now you’re wondering, “when is my due date?” Because many women don’t know exactly when they ovulated, or when they conceived, typically a due date calculator will estimate your due date from the last menstrual period (LMP). When using the LMP, your due date is estimated to be 40 weeks after the first day of your LMP. Additionally, your cycle is assumed to be 28 days long with ovulation occurring at day 14, according to Naegele’s rule which adds 280 days (40 weeks) onto the LMP. However, there are other ways of calculating a due date that may be more accurate.

What’s Naegele’s Rule?

Named after a German Obstetrician in the early 1800’s,  Naegele’s rule is what most due dates are based on. It predicts childbirth to occur 280 days after the first day of the last menstrual period. This rule is widely used but may not be the most accurate way to figure out your estimated due date. For one thing, the rule doesn’t take into account a woman’s unique cycle length; it assumes a cycle length of 28 days and a luteal phase length of 14 days but cycles deviate from that number quite a bit.

What’s the Mittendorf-Williams rule?

In a study done in 1990, researchers discovered that pregnancy lasted an average of 288 days past LMP for Caucasian first time moms. For Caucasian women who were not first time moms, their date of delivery averaged at about 283 days past LMP (3 days after Naegele’s rule predicted).While Naegele’s rule is most widely used for calculating your pregnancy due date, the Mittendorf-Williams rule is proving more accurate. It takes into account much more information about the mother, her habits, and her history such as coffee consumption, age, and past prenatal history.

What is an estimated due date?

An estimated due date (EDD) is a “best guess” as to when baby might be born based on a conception calculator. However, only about 5% of babies are born on their due date while most of the rest of babies are born within the window of two weeks before an two weeks after your estimated due date.

How are the weeks of pregnancy calculated?

Because the pregnancy calculator will estimate the due date from the first day of the last menstrual period, if you have a 28 day cycle and get a BFP on the day you should have gotten your period, you are already considered 4 weeks pregnant. That means that you have completed your 4th week of pregnancy (even though you only conceived 2 weeks ago, I know, confusing). The following day you are considered to be 4 weeks 1 day pregnant and so on until you complete the 5th week and can say you are 5 weeks pregnant.

How do you figure out how many weeks pregnant you are?

To find out how many weeks pregnant you are you count how many weeks it’s been since your LMP. Even though you aren’t actually pregnant for the first two weeks of your “pregnancy” this is the way pregnancy is calculated based on Naegele’s rule. The reason pregnancy is calculated this way is because most women don’t know the exact day they conceive so timing from the LMP is the best way to determine the EDD.

However, as we mentioned earlier, Naegele’s rule doesn’t come without flaws. For one thing it is based on the idea that all women have a 28 days cycle with ovulation occurring on day 14 (a 14 day luteal phase). Clearly this isn’t the case for all women. Studies have found that women tend to have their babies a few days after the due date on average. Studies are finding that the Naegele rule consistently places the EDD about 2-4 days too early. So a better estimate may be 40 weeks and 3 days from LMP or to go with the Mittendorf-Williams rule instead.

How do you calculate due date from conception?

If you know when you conceived you can use the dude date calculator to calculate your due date by adding 38 weeks to the date of conception. This due date may be more accurate than a LMP due date calculation if you have irregular or consistently longer or shorter cycles than 28 days.

How do you calculate due date by date of intercourse?

Sperm can live in the vagina for up to 4 days. That means if you have sex on monday you may conceive on Monday, Tuesday, Wednesday, Thursday, or even Friday. To come up with a date take the average, 2 days, and assume that conception happened 2 days after sex.

What is the date of conception?

The date of conception is the day that the egg and sperm meet. the reason this is so hard to pinpoint is that sperm can live in the vagina for 4 days and the ovum (egg) can live for about 24 hours after being released, that give a 5 day window where you could potentially get pregnant. Add in not knowing exactly what day you are ovulating and the potential conception window widens.

What is a Due Month?

When you’re given a due date by your practitioner or a due date calculator you’re also told that it’s normal for babies to come as much as 2 weeks before and 2 weeks after your EDD. Because of this and because there is no way to know for sure the exact date of delivery, many women choose a due month instead of a fixed day for expecting labor to begin. One reason is that it helps eliminate some of the stress and fear associated with going past your due date. Your due month is simply the two weeks before and two weeks after your EDD given by your practitioner of the due date calculator. With a due month it may be easier for you to trust your baby to come when the time is right (barring any complications, of course).

What is “gestational age”?

Gestational age (GA) is the term used to describe how far along the pregnancy is and how long baby has been gestating (growing in the uterus). The due date calculator helps you figure this out. However, as we said earlier, gestational age is counted from the first day of the LMP. Therefore, when baby is conceived (around 2 weeks after LMP) he is considered to be already 2 weeks gestational age. Conceptional age (CA) is the age of baby from when he was conceived.

What does GA stand for in an ultrasound?

If you get an ultrasound you may notice a “GA” on the image with a number of weeks and days. This stands for, you guessed it, gestational age but is based on how the baby is measuring, not on your LMP, which the due date calculator uses. It’s normal for these dates to not match up completely but if they are significantly different your doctor may want to dig a little deeper to figure out the real conception date, and perhaps may change your due date based on the ultrasound gestational age. Early ultrasounds are very accurate when dating a pregnancy and can be helpful if you don’t know your LMP or your periods are irregular.  You don’t have to have an early ultrasound, especially if you are fairly certain of your cycle length and conception window. There are many things to think about when weighing the pro and cons of ultrasounds. A study shows that early dating ultrasounds don’t change the incidence of induction so an early ultrasound may not be of much use.

What is a LMP date?

LMP simply stands for last menstrual period. The last menstrual period is the last period you had before becoming pregnant. Your LMP date is the first day of your last menstrual period. This date is used by the due date calculator to figure out your due date (by adding 40 weeks according to Naegele’s rule).

How are the trimesters of pregnancy divided?

Pregnancy is divided into three trimesters. The first trimester is from conception to the end of week 12. The second trimester is from week 13 to week 26 and the third trimester is from week 27 to birth.

The first trimester, though a similar length as the other two, feels much shorter because for the first 4+ weeks you don’t know you’re pregnant. The second trimester is often a nice break from the nausea, fatigue, and, well, just feeling fat of the first trimester. Many women find the second trimester to be the most comfortable. Though you’ll likely start showing in the second trimester (if not earlier) the third trimester is when you’ll be obviously pregnant and may begin to get uncomfortable. But all of the discomfort of the third trimester seems to be by perfect design because by the end of the third trimester you’re more than ready to give birth!