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How Does the Doctor Calculate Your Due Date?

By Sharon Perkins ; Updated June 13, 2017

Naegels Rule

The time honored method for your doctor to assign a due date is to take the first day of your last menstrual period, or LMP, count back three months and add seven days. For example, if the first day of your last menstrual period was April 1, count back three months to January 1, add seven days, and your due date is January 8. This is known as Naegels rule, Healthline states.

There are problems with this method; many women have no idea when they’re last menstrual period started, especially if their periods are irregular. This method is also based on an “average” menstrual cycle of 28 days and ovulation on day 14, which doesn’t always occur. Even women who are regular may have a month where they ovulated earlier or later than expected. For example, a woman who usually ovulates on day 14 is sick and doesn't ovulate until day 21; her due date will be one week later than Naegels Rule suggests. A woman who ovulates earlier than 14 days will be due earlier than the date Naegels rule suggests.


Ultrasound is by far the most accurate way for your doctor to calculate your due date, especially if it’s done early in the pregnancy, around six weeks. Fetal development is quite standardized at this point, within a few days; the gestational sac is first seen at five weeks, the yolk sac, which nourishes the baby at five-and-a-half-weeks, and the development, the fetal pole, at six weeks. The heart beat usually appears by six-and-a-half to seven weeks, the American Pregnancy Association (APA) states.

Many women do not have early ultrasounds, unless they’re having problems, such as bleeding, or have undergone assisted reproductive technology procedures such as in vitro fertilization. Many first ultrasounds are done around 18 to 21 weeks, to assess for fetal abnormalities, according to the March of Dimes. However, doctors can use second trimester ultrasounds to date a pregnancy within 10 to 14 days accuracy, Healthline states.

Human Chorionic Gonadopin Levels

Human Chorionic Gonadopin, or hCG, is a hormone produced by the placenta. Your doctor can do a blood test to check your hCG levels. HCG can be detected in the blood by the fourth week of pregnancy, or about two weeks after embryo implantation. There’s a wide range of variability in hCG levels, from 18 to 7,340 milli-International Units per millilter (mIU/ml) at week five, according to the American Pregnancy Association, which makes hCG levels an unreliable determination of exact dates, although early levels can allow your doctor to pinpoint a pregnancy within a week or two.

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