Figuring out your due date seems like a straightforward process. Physicians traditionally focus on the expectant mother’s last menstrual period to make this determination. In this technological age, an array of user-friendly due date calculators are also available, which make it easier and more fun for expectant mothers to keep track of their pregnancy and document important milestones. But how much should you trust your estimated due date?

Basic Guidelines

On average, the typical pregnancy lasts about 40 weeks or 280 days. The most common way to determine a due date is using a woman’s last monthly menstrual period (LMP). Physicians may use a calculating method called Naegele’s Rule. Once they determine the LMP, they count back three calendar months from that date, then add one year plus seven days to the date to determine an estimated due date.


Problems With LMP Calculations

The issue with the accuracy of this type of due date calculation is that it is based on an average-length menstrual cycle of 28 days. However, as any woman who's discussed her menstrual cycle with friends knows, they vary widely. A typical cycle can fall anywhere between 21 and 35 days. Additionally, many women do not usually recall the exact date of the first day of their last period. Instead, they use an approximate date and this can hinder the accuracy of the calculation.


Calculation by Conception

Women can only conceive when they are ovulating. The window of fertility begins three to five days before ovulation — because sperm can survive until the egg arrives — and continues for one to two days afterward. When women know the exact date of conception, they can get an estimated due date by counting 38 weeks, the length of human gestation, from the conception date. This method works best for women who keep detailed records of their ovulation cycles along with the dates on which they had sex during those cycles.


Issues with Conception-Based Calculations

Conception does not always occur on the day that a woman has sex, unless she was ovulating. It occurs when the egg is fertilized. However, once the sperm reach the fallopian tubes, they can live up to five days waiting for an egg to fertilize. Fertilization can occur in as little as 30 minutes or take 72 hours. A woman can conceive up to five days after having sex, so the true date of conception is difficult to pinpoint.


Calculating by Ultrasound

According to the American College of Obstetricians and Gynecologists, the most accurate method of due date estimation is an ultrasound measurement of the embryo and fetus in the first trimester. If the LMP date is reliable, studies show that there is little difference in the accuracy of a prediction for the date of birth.


Multiple Birth Date Calculator

In most cases, multiple births occur about two weeks earlier than single births. However, when calculating the due date, it remains the same as it would if the mother were having one child. The LMP, conception date, and day of fertilization do not change the due date for multiple births. These births can occur between 35 and 38 weeks of gestation. If the physician uses a sonogram to calculate a due date, they determine the EDD or estimated date of delivery according to the size of the larger twin.


IVF Due Date Calculators

For women who have undergone in vitro fertilization (IVF), due date calculations are much more exact. The egg retrieval or insemination date is the date of ovulation. Some women have a transfer of a three-day or five-day embryo. If it is a three-day, the doctor determines a due date of 263 days from the transfer, and for a day-five embryo, they add 261 days to the transfer date.


Online Due Date Calculators

Online due date calculators can provide an EDD before a woman consults with her obstetrician or doula for the first time. The accuracy of these apps depends on the accuracy of the LMP entered. The National Health Service, the Mayo Clinic, and the March of Dimes websites provide free due date calculators. The MD+Calc also provides the fetus’ gestational age and date of probable conception.


Phone App Due Date Calculators

Most of the due date calculator phone apps also provide options for tracking the pregnancy. These apps allow the counting down and documentation of the months leading up to childbirth. Others assist with the steps leading up to pregnancy, such as ovulation tracking. Some have notification reminders for OB/GYN appointments. Medical communities say that these apps are not replacements for professional prenatal care.


The Importance of Accurate Due Dates

Only a small percentage of women deliver on their due date. However, calculating an accurate EDD is crucial not only for the health of the mother but for her unborn child. This date lets healthcare providers schedule screenings at the proper times and assess gestational age, which can help prevent preterm births, post-term births, and other complications.


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