Due Date Calculator

How Is a Due Date Calculated?

Your due date, also known as the estimated date of delivery (EDD), is a predicted time frame for when your baby might be born. Although this date gives you a good idea of what to expect, only about 4% of babies are actually born on their exact due date. That’s why it’s often more helpful to think of a range rather than a single day.

Several methods can be used to estimate your due date, including:

• The first day of your last menstrual period (LMP)

• Ultrasound measurements

• The conception date

• The date of an IVF embryo transfer

Using the Last Menstrual Period (LMP)

This is the most common method used in due date calculators. It assumes a standard 40-week pregnancy (280 days) starting from the first day of your last period, not the date of conception. While there’s some debate over when pregnancy technically begins—whether at fertilization or implantation—the gestational age always counts from the LMP.

Most pregnancies last between 37 and 42 weeks, so calculating 40 weeks from the LMP provides a reliable average for determining your expected delivery window.

Estimating Due Date with Ultrasound

An ultrasound scan is another accurate way to estimate your due date, especially in the early stages of pregnancy. It works by measuring the baby’s size and comparing it to typical growth patterns. This method is non-invasive, safe for the baby, and can provide a fairly precise estimate when performed during the first trimester.

Due Date Based on Conception

If you know your exact conception date, this can also help determine when your baby is due. However, conception typically occurs about two weeks after the start of the last menstrual period, so dates calculated from conception are usually about two weeks later than those based on LMP.

In Vitro Fertilization (IVF) Due Date Estimates

In the case of IVF pregnancies, the due date is more exact because the embryo transfer date is known. Most due date calculations for IVF still use the standard 40-week model, but the reference point could be:

• The egg retrieval date

• The date of ovulation or insemination

• The embryo transfer date (either day 3 or day 5)

This calculator uses the embryo transfer date to provide a more precise prediction.

Understanding the Due Date Range

While your due date marks a significant milestone, it’s important to understand that delivery anytime between 37 and 42 weeks is usually considered normal. Here’s how those ranges break down:

• Early Term: 37–38 weeks

• Full Term: 39–40 weeks

• Late Term: 41 weeks

• Postterm: 42 weeks or later

• Preterm: Before 37 weeks

Most babies born within the early to late-term window are healthy, but full-term births typically result in the best outcomes. If labor begins too early—for example, at 33 weeks—doctors may try to delay it to allow the baby more time to develop. On the other hand, if pregnancy continues past 42 weeks, doctors may recommend inducing labor to avoid risks.

One concern with postterm pregnancies is that the placenta may start to deteriorate, reducing its ability to deliver oxygen and nutrients. Since the baby continues growing and needs more support, this mismatch can cause complications if the pregnancy is not managed carefully.

Refrences:

1. Moore, Keith. “How accurate are ‘due dates’?” BBC News, 2015.

2. Wikipedia. “Pregnancy.” 2017.

3. PubMed Health. “Pregnancy and birth: When your baby’s due date has passed.” 2014.