How accurate is the due date calculation?

Estimated due dates are approximations. Only about 4% of babies are born on their exact due date. Most babies (about 80%) are born within the two weeks before or after the due date. The calculation is a standard medical estimate based on a 280-day gestational period.

Why does the calculator use the last menstrual period?

Doctors typically date pregnancies from the first day of your last menstrual period (LMP) because most women know this date, whereas the exact date of conception is often unknown. This method assumes conception occurred about 2 weeks after your LMP.

What if I know my exact conception date?

If you know your exact conception date (for example, if you were tracking ovulation), you can select 'Conception Date' in the calculator for a more precise estimate based on fetal age rather than gestational age.

How is IVF due date calculated?

For IVF pregnancies, the due date is calculated based on the transfer date and the age of the embryo at transfer (usually 3 or 5 days). This is often more accurate than LMP calculations since the fertilization timing is known.

What are the pregnancy trimesters?

Pregnancy is divided into three trimesters: First Trimester (Week 1-13), Second Trimester (Week 14-26), and Third Trimester (Week 27-40+). Each stage brings different fetal development milestones and maternal changes.

Maternal Health

Pregnancy Calculator

The Pregnancy Calculator can estimate a pregnancy schedule based on the provided due date, last period date, ultrasound date, conception date, or IVF transfer date.

Last updated: January 2026

Supports IVF and Ultrasound dating

Estimates Trimester and Gestational Age

Based on medical standards

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Pregnancy Calculator

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Pregnancy Term & Due Date

Pregnancy is a term used to describe a woman's state over a time period (~9 months) during which one or more offspring develops inside of a woman. Childbirth usually occurs approximately 38 weeks after conception, or about 40 weeks after the last menstrual period. The World Health Organization defines a normal pregnancy term to last between 37 and 42 weeks. During a person's first OB-GYN visit, the doctor will usually provide an estimated date (based on a sonogram) at which the child will be born, or due date. Alternatively, the due date can also be estimated based on a person's last menstrual period.

While the due date can be estimated, the actual length of pregnancy depends on various factors, including age, length of previous pregnancies, and weight of the mother at birth. However, there are still more factors affecting natural variation in pregnancy terms that are not well understood. Studies have shown that fewer than 4% of births occur on the exact due date, 60% occur within a week of the due date, and almost 90% occur within two weeks of the due date.

Pregnancy Detection

Pregnancy can be detected either by using pregnancy tests or by the woman herself noticing a number of symptoms, including a missed menstrual period, increased basal body temperature, fatigue, nausea, and increased frequency of urination.

Pregnancy tests involve the detection of hormones that serve as biomarkers for pregnancy and include clinical blood or urine tests that can detect pregnancy from six to eight days after fertilization. While clinical blood tests are more accurate, and can detect exact amounts of the hormone hCG (which is only present during pregnancy) earlier and in smaller quantities, they take more time to evaluate and are more expensive than home pregnancy urine tests.

Pregnancy Management

There are a number of factors that need to be considered during pregnancy, many of which are highly dependent on the individual's situation, such as medication, weight gain, exercise, and nutrition.

Medication

Taking certain medications during pregnancy can have lasting effects on the fetus. In the U.S., drugs are classified into categories A, B, C, D, and X by the Food and Drug Administration (FDA) based on potential benefits vs. fetal risks. A person that is pregnant should consult their doctor regarding any medications they plan to use during their pregnancy.

Weight gain

Weight gain is a largely inevitable and necessary aspect of pregnancy that varies between people. It affects many aspects of fetal development, such as the weight of the baby, the placenta, extra circulatory fluid, and its fat and protein stores. The Institute of Medicine recommends an overall pregnancy weight gain of 25-35 pounds for women who are considered "normal" weight (BMI 18.5-24.9). Check our Pregnancy Weight Gain Calculator for more details.

Exercise

Studies indicate that aerobic exercise during pregnancy helps to improve or maintain physical fitness as well as possibly decreasing the risk of C-sections. Although it varies between women, regular aerobic and strength-conditioning exercise are often recommended. Nevertheless, caution is advised, and a pregnant woman should consult their doctor if any symptoms present.

Nutrition

Nutrition during pregnancy is particularly important for the health of the mother and baby. Pregnancy requires different nutritional considerations than a person would have in a non-pregnant state due to increased energy and specific micronutrient requirements.

Certain vitamins such as Vitamin B9, also known as folic acid, can help decrease the risk of certain defects, while other nutrients such as DHA omega-3 that is necessary for proper brain and retinal development cannot be produced efficiently by infants.