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Should You Use a Pregnancy Calculator?

You’ve just found out you're pregnant and congratulations are in order. This is a very joyous moment for you and your family. After the initial excitement and jumping for joy, your first question may be “when is my baby due?”

Even though only about 1 in 20 people give birth on their estimated due date, it’s important to know when your baby is expected. Your due date is used to track your baby’s growth, as well as when to perform certain diagnostic tests. Pregnancy due date calculators are available online and in apps for your phone – but should you use one?

Let’s discuss the different ways your estimated due date is determined and how pregnancy calculators work.

How Do Pregnancy Calculators Work?

Pregnancy is measured by weeks and your estimated due date is when you reach 40 weeks. Most women give birth anywhere between 38 and 42 weeks. It’s a bit of a misnomer to say you’re pregnant for 40 weeks though – you’re actually only pregnant for 38 weeks. Two of the weeks included are the first two weeks of your menstrual cycle when your body is preparing to get pregnant – a follicle is becoming a mature egg to be released by your ovary and the lining of your uterus is building back up.

Pregnancy calculators give you an estimated due date based on your last period, the average length of your cycles, date of conception, or when you had your IVF transfer.

How Is Due Date Calculated?

There are several ways to calculate your estimated due date:

  • Due Date Based on Last Period
  • This is the most common way an estimated due date is calculated. The calculator will ask you for the date of the first day of your last period and what the average length of your menstrual cycles is. It gives you your estimated due date, which is 40 weeks (280 days) from the start of your last period.

    If you have irregular periods, the calculator adjusts for this by asking you what the average length of your cycle is. If your average cycle length is longer than 28 days, your estimated due date is pushed farther out. If your cycle is shorter, then the date is moved closer. 

  • Due Date Based on Conception
  • Once fertilization or conception occurs, your estimated due date is then calculated by counting out to 38 weeks (266 days). This method is not as accurate as your last period because it’s hard to know the exact time you ovulated. Also, sperm can survive in your reproductive tract for up to 5 days, making it hard to pinpoint the exact day the egg was fertilized.

  • Due Date Based on in-vitro fertilization (IVF) transfer
  • During IVF, sperm and an egg are joined in the lab to form an embryo. The embryo is allowed to grow for the next 3 to 5 days and then it’s transferred to your uterus for implantation (fingers crossed!). If the embryo was transferred on day 3, your estimated due date will be counted out to day 263. If it’s a day 5 transfer, your due date is 261 days out.

  • Due Date by Ultrasound
  • Outside of IVF transfer, a first trimester ultrasound is the most accurate way to get an estimated due date. An ultrasound measures the size of the fetus to give a more accurate date of conception and ultimately, an estimated due date. Ultrasounds done later in pregnancy are not as accurate at predicting due date.

    How Accurate Are Pregnancy Calculators?

    There’s no method of due date calculation that’s 100% correct. That’s why it’s called an estimated due date. There are many factors that affect your due date, like your age, your medical history, and how many babies you’re carrying.

    According to ACOG, the most accurate way to calculate your estimated due date is by an ultrasound during your first trimester. Any app or online calculator you use is only going to be as accurate as the information you give it. It’ll determine when you should be at 40 weeks, but if you aren’t sure when your last period was or you haven’t been tracking the length of your cycles, the date might be way off.

    Should I use a pregnancy calculator?

    A pregnancy calculator is a great method to give you an idea of how far along you are once you get a positive pregnancy test. Then you can schedule your first appointment with your healthcare provider. They’ll do an ultrasound and see how your estimated due date matches with the ultrasound’s information.

    What Should I Do Next?

    If you’ve taken an at-home pregnancy test and it’s positive, it’s time to make your first prenatal appointment. Your first appointment should be before 10 weeks. This way your healthcare team can check the baby for an estimated due date and plan your course for care over the next 9 months!

    Whether you’re preparing to get pregnant or just found out you are, it’s always a good idea to take a prenatal vitamin and boost your immune system. Pink Stork’s Total Prenatal + DHA has all the vitamins and nutrients you need to support a healthy pregnancy. And our Prenatal Probiotics support a healthy gut and immune system to protect both you and your baby.


    America College of Obstetricians and Gynecologists. (2022). Methods for estimating due date.

    Flo. (2022). Pregnancy Due Date Calculator.

    Forbes Health. (2023) Due Date Calculator: When is Your Baby Due?