How Do You Determine Your Pregnancy Due Date

Lmp Versus Early Ultrasound

How To Calculate Your Pregnancy Due Date

The accuracy of using your LMP to date your pregnancy depends on having regular cycles and assumes you ovulated on day 14 of your cycle and conceived right away – none of which may be true. Because few women know the exact day they ovulated or conceived, an ultrasound done in the first trimester of pregnancy has been shown to the be the most accurate way to date a pregnancy.

If an ultrasound date in the first trimester differs from your LMP date by seven days or more, we would go with the ultrasound. If the ultrasound date is within seven days of your LMP date, we would stick with your LMP date. Ultrasounds done later in the pregnancy are less accurate for dating, so if your due date is set in the first trimester, it shouldn’t be changed.

However, if you did not get a first trimester ultrasound for dating and an ultrasound done to look at the baby’s anatomy differs from your LMP date by 10 to 14 days, we would change your date to the ultrasound estimate.

Conception Calculator: When Did I Conceive

As soon as you get a positive pregnancy test, the first thing you probably want to do is type conception calculator or When did I conceive? into Google. And if you do, youll find loads of online calculators that promise to give you your conception date. But the truth is that online calculators will only give you an estimate of your conception date.

Before we dive into understanding the complex process of conception, lets take a look at how these conception calculators typically work.

When youre trying to determine when your pregnancy began, the date youre really looking for is implantationwhen the fertilized egg burrowed into your uterine lining, thereby beginning your pregnancy. You can figure out this date using a couple of simple formulas .

If you know the date you ovulated, add 9* days.

Ovulation date + 9 days =

If you know the first day of your last menstrual period , add 23** days.

Date of LMP + 23 days = Implantation date

*The average viable embryo implants at 9 days post ovulation.

**14 + 9 = 23 days.

Again, its important to note that the above formulas are just a rough guess based on when ovulation and implantation tend to happen on average for most women. For accurate information about your specific fertile window, tracking your cycle is really helpful.

Are There Some Things That Can Make Your Pregnancy Longer

In 2013, Oberg et al. published a large study that looked at more than 475,000 Swedish births, most of which were dated with an ultrasound before 20 weeks. They found that genetics has an incredibly strong influence on your chance of having a birth after 42 weeks:

  • If youve had a post-term birth before, you have 4.4 times the chance of having another post-term birth with the same partner
  • If youve had a post-term birth before, and then you switch partners, you have 3.4 times the chance of having another post-term birth with your new partner
  • If your sister had a post-term birth, you have 1.8 times the chance of having a post-term birth

Overall, researchers found that half of your chance for having a post-term birth comes from genetics. This includes the babys genetic tendency to gestate longer , and the mothers genetic tendency to carry a pregnancy longer. The Swedish researchers even proposed that you could call some pregnancies resistant, because these mothers and/or fetuses have a genetically decreased tendency to start labor.

Other factors that may make your pregnancy more likely to go longer include:

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Conception Calculator Based On Due Date

So, what day did I conceive? There are two ways to use our conception calendar calculator.

The first way is to provide the birth due date. When you visit the doctor they will most likely perform an ultrasound. Although the doctor can see the baby from the 5-6th week of pregnancy, the best time to go for the first sonography is between week 8 and 12.

Based on the characteristics of the image and the results of your blood test, the due date and week of pregnancy can be determined. Remember that those dates, as well as conception calendar, are only an estimation and may change during the next visit.

Pregnancy typically lasts around 40 weeks . That is 266 days from the conception date. If a baby is born before week 37, it’s called a premature birth. But don’t worry if the due date changes or you go in labor on a different date – only 4% of babies are born on their due date and 80% are born either two weeks before and after the date.

Let’s go back to the question: what day did I conceive? In our conception calculator, we subtract 266 days from the estimated due date and get the most probable day of conception:

due date – 266 = conception date

Why Is This Method Wrong

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This method does not work because many people are induced when they reach 39, 40, 41, or 42 weeks.

If you do include these induced people in your average, then you are including people who gave birth earlier than they would have otherwise, because they were not given time to go into labor on their own.

But this puts researchers in a bind, because if you exclude a person who was induced at 42 weeks from your study, then you are ignoring a pregnancy that was induced because it went longerand by excluding that case, you artificially make the average length of pregnancy too short.

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What Is The Date Of Conception

Conception occurs after ovulation, when the sperm meets the egg. For instance, if you had intercourse on the 5th of the month, and you did not ovulate until the 8th of the month, the conception will occur on or just after the 8th. If you have a 28 day-cycle, conception is known to occur between 11 and 21 days after the first day of your LMP. As most of us do not know the date of conception, it is taken as an estimate based on the LMP method.

How To Calculate How Many Weeks You Are

  • Start by counting the recurring day of your last menstrual period
  • Stop counting recurring days when you reach the present time.
  • The number of recurring days you count is the number of weeks you are now.
  • Example:count one week to the nextuntil you reach the present date.

    LMP = December 2, 2021

    1st Wed = December 9, 2021 = 1 Week2nd Wed = December 16, 2021 = 2 Weeks9th Wed = February 3, 2022 = 9 WeeksYou are 9 Weeks and 1 Day pregnant

    Another method to calculate how many weeks pregnant you are is to:

  • Count the days from the first day of your Last Menstrual Period until you reach today’s date.
  • Now divide the counted days by 7.
  • The answer will be how many weeks you are now.
  • How Many Weeks Am I Formula:LMP = Last Menstrual PeriodHow Far Along Am I Formula:LMP = Last Menstrual Period*LMP date may be adjusted if unknown or following ultrasound.

    Each of these methods will give you the gestational age of your pregnancy.

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    How Are Due Dates Calculated

    If you were having regular periods before pregnancy, your doctor will calculate your due date based off of your last menstrual period. This goes back to the fact that in order to get pregnant, your body ovulatedor released an eggroughly in the middle of your cycle and it was fertilized by sperm. That was the moment of conception.

    To clarify, the gestational period of 40 weeks actually starts with the first day of your last period, which adds two weeks of time to the gestational period when your baby didnt even exist yetclear as mud, right?Is an ultrasound a more accurate way of finding out my due date?If you are having irregular cycles before getting pregnant, an ultrasound is usually done to find out how far along you are. An ultrasound is actually the most accurate way to date a pregnancy because all fetuses grow at a consistent rate during the first trimester and early second.

    In other words, if your baby measures 9 weeks 2 days when you have your ultrasound, thats how far along you are, no matter when your last period was. Some women with regular cycles are confused about why their ultrasound due date doesnt match up with their last menstrual period due date. Ovulation isnt a perfect science and can happen earlier or later than the norm, which might shift your due date slightly.

    In A Typical Pregnancy

    How to Use Pregnancy Wheels to Calculate Your Due Date

    For a woman with a regular period, conception typically occurs about 11-21 days after the first day of the last period. Most women do not know the exact date of conception because it can be challenging to know exactly when ovulation occurs. Thus, their conception date is merely an estimate based on the first day of their last period.

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    Dating Pregnancy Using Ultrasound

    Ultrasound in the first trimester is the most accurate method of dating pregnancy. During the scan, baby is measured and the age of pregnancy is estimated by comparing baby’s size against recorded averages. If there is a significant difference between gestational age according to LMP and gestational age according to the ultrasound, you may be given a new gestational age and due date based on the ultrasound measurements.

    Your pregnancy may also be dated by ultrasound measurement if an LMP formula was not appropriate for your pregnancy for any reason. LMP formulas are not suitable in several situations including the following: LMP is not known or is not certain, birth control pills were taken in the two months before LMP, the menstrual cycle is irregular, you are carrying multiples .

    Although the age of pregnancy based on ultrasound measurements is more accurate than the age of pregnancy based on LMP, both are estimates and not definitive statements.

    What Is Gestational Age Can It Be Different Than What The Calculator Shows

    Gestational age is the term used to describe how far along the pregnancy is and how long baby has been gestating .

    If you get an ultrasound you may notice a GA on the image with a number of weeks and days. This figure is based on how the baby is measuring, not on your LMP, which the due date calculator uses.

    Its normal for these dates to not match up perfectly. If there are significant differences in the dates, your doctor may want to dig deeper to determine conception date. As a result, your midwife or doctor 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 dont know your LMP or your periods are irregular.

    Note that you dont have to have an early ultrasound, especially if you are fairly certain of your cycle length and conception window. This study shows that early dating ultrasounds dont change the incidence of induction.

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    Theres A New Way To Calculate Your Pregnancy Due Date

    When are you due? When you are pregnant, a certain date becomes very important. It is a date you plan things around, and a date that you know will change everything. Traditionally due dates are calculated by last menstrual period. But now, doctors have new guidelines to ensure that women are receiving the most accurate due date possible.

    Currently, due dates are calculated by using an estimate based off of a womans last period and the average length of pregnancy, which is a total of 40 weeks. Assuming a regular, 28-day cycle for a woman, ovulation usually occurs around day 14 of the cycle so if fertilization occurs during that window, a woman is considered already two weeks pregnant, as the due date is calculated from day one of the cycle. By the time most women find out they are pregnant when they miss their periods two weeks later, they are already considered a month pregnant.

    The problem, of course, is that not every womans cycle operates on a regular 28-day pattern, and ovulation can be unpredictable and change as a result of stress, birth control or sleep deprivation. A woman who has a longer cycle and ovulated later but didnt realize it could actually have a due date thats off by a week hence the reason doctors usually say theres a good two-week cushion for calculating due dates. And lets face it, sometimes women do not recall exactly the date of their last period.

    Disclaimer About The Calculator

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    Our calculator strives for the highest accuracy, but your pregnancy progress may differ from the calculator’s results for several reasons including possible miscalculation errors in input data individual variations in the timing of ovulation, and/or conception.

    The information provided regarding pregnancy progress and fetal development is a guide through the weeks. It is not specific information for a specific individual but serves as a general guide only. As each pregnancy differs, any dates, weights, and lengths should be considered general information for interest only.

    Our week-by-week guides use both standard and metric units of measure including pounds and kilograms.

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    How Do You Calculate Your Due Date

    One simple and common method for calculating your delivery date is as follows:

    • Mark down the date when your last period started.
    • Add seven days.
    • Count back three months.
    • Add a whole year.

    For example, if the first day of your last period was April 9, 2019, adding seven days will make the date April 16, 2019. Counting back three months results in the date of January 16. Finally, adding a year yields an estimated delivery date of January 16, 2020.

    The above strategy is based on the assumption that conception occurred 14 days after the start of your last menstrual period. Also, it’s important to remember that no matter what “pen and paper” calculations you use to estimate your delivery date, these methods are just that estimates. Most babies are born between 38 and 42 weeks and only a small percentage of women actually deliver on their estimated due date.

    Your doctor can use other methods to provide you with your delivery date, such as conducting an ultrasound examination in the first trimester. A physical exam also can be done.

    So What Is The Risk Of Stillbirth As You Go Past Your Due Date

    Since the late 1980s, there have been at least 12 large studies that looked at the risk of stillbirth during each week of pregnancy. Some of the researchers used open-ended stillbirth rates, and some of them used actual stillbirth rates.

    All of the researchers found a relative increase in the risk of stillbirth as pregnancy advanced.

    To get an accurate picture of stillbirth in people who go past their due date, it would be best to look at studies that took place in more recent times. Ive chosen 3 of the most recent studies to show you from Norway, Germany, and the U.S. To see all of the other studies, click to view the entire table here.

    All 3 of these studies used the actual stillbirth ratenot the open-ended stillbirth rate. Two studies used ultrasound to calculate gestational age, and one study used the LMP.

    The largest meta-analysis to date on risks of stillbirth and newborn death at each week of term pregnancies was published in 2019 . A meta-analysis is when researchers take multiple studies and combine all the data together into one big meta study. The researchers included 13 studies . All of the studies were conducted in countries defined as high-income by the World Bank.

    The researchers also found evidence that health care systems are failing Black mothers and babiesan alarming but common theme in health care research. Black mothers were 1.5 to 2 times more likely than White mothers to have a stillbirth at every week of pregnancy.

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    What Does It Mean To Be Full Term

    For many years, a baby was defined as being born at term if it was born between 37 weeks 0 days and 41 weeks 6 days. Anything before that 5-week period was considered preterm, and anything after those five weeks was post-term.

    Over time, though, research began to show that health problems were more common at certain points during this 5-week term period. In particular, newborns are more likely to die if they are born before 39 weeks, or after 41 weeks.

    The chance of a newborn having problems is lowest if he or she is born between 39 weeks and 0 days and 40 weeks and 6 days .

    In 2012, a group of experts came together to define term pregnancy. Based on their review of the research evidence, they broke the 5-week term period into separate groups .

    • Early term babies are born between 37 weeks 0 days and 38 weeks 6 days
    • Full term babies are born between 39 weeks 0 days and 40 weeks 6 days.
    • Late term babies are born between 41 weeks 0 days and 41 weeks 6 days
    • Post term babies are born at 42 weeks and 0 days or later

    This Signature Article focuses on the evidence on determining due dates. To read our separate Signature Article all about the Evidence on Inducing Labor for going past your Due Dates, .

    What Is Naegele’s Rule

    Your Pregnancy: Due Date Calculator | Parents
  • Add seven days
  • Move the date back three months
  • For example, if the first day of your last period was August 11, 2019, you would:

    • Add 1 year
    • Add 7 days
    • Move the date back 3 months
    • Your due date would be May 18, 2020.

    If your fertility app gives you a different date than the one calculated by your doctor, it most likely because it was based on the ovulation date and not the date of your last period. If you ovulated earlier or later than day 14 of your cycle, this would shift your due date accordingly.

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