You just found out you are pregnant and congratulations are in order. This is a very happy time for you and your family. After the initial excitement and jumping for joy, your first question may be “when is my baby due?”
Although only about 1 in 20 people give birth on their estimated due date, it’s important to know when your baby is due. Your due date is used to monitor your baby’s development, as well as when you should have certain diagnostic tests. 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 in weeks and the estimated due date is when you reach 40 weeks. Most women give birth between 38 and 42 weeks. It’s a bit of a misnomer to say you’re 40 weeks pregnant though – you’re actually only 38 weeks pregnant. Two of the weeks included are the first two weeks of your menstrual cycle when your body prepares to become pregnant – a follicle becomes a mature egg that is released from your ovary and the lining of your uterus is rebuilt.
Pregnancy calculators give you an estimated due date based on your last period, the average length of your cycles, your conception date or when you had your IVF transfer.
How is the due date calculated?
There are several ways to calculate the estimated due date:
This is the most common way to calculate an estimated due date. The calculator will ask you the date of the first day of your last period and what the average length of your menstrual cycles is. It gives you the 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 your average cycle length is. If your average cycle length is longer than 28 days, the estimated due date is shifted further. If your cycle is smaller, then the date is moved closer.
Once fertilization or conception occurs, the estimated due date is then calculated by counting up to 38 weeks (266 days). This method is not as accurate as your last period because it is difficult to know the exact time you ovulated. Also, sperm can survive in your reproductive tract for up to 5 days, making it difficult to pinpoint the exact day the egg was fertilized.
During IVF, sperm and an egg unite in the laboratory to form an embryo. The embryo is allowed to develop for the next 3 to 5 days and is then transferred to your uterus for implantation (fingers crossed!). If the embryo was transferred on day 3, the estimated due date will be calculated up to day 263. If it is a day 5 transfer, the due date is 261 days.
Other than an 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 in predicting the due date.
How accurate are pregnancy calculators?
There is no method of calculating the due date that is 100% correct. That’s why it’s called an estimated due date. There are many factors that affect your due date, including your age, your medical history, and how many babies you are carrying.
According to ACOG, the most accurate way to calculate your estimated due date is with an ultrasound during the first trimester. Any app or online calculator you use will only be as accurate as the information you give it. It will determine when you should be at 40 weeks, but if you’re not sure when your last period was or you’re not tracking the length of your cycles, the date may be too far off.
Should I use a pregnancy calculator?
The pregnancy calculator is a great method to give you an idea of how far along you are once you get a positive pregnancy test. You can then schedule your first appointment with your healthcare provider. They will do an ultrasound and see how your estimated due date matches the ultrasound information.
What should I do next?
If you’ve taken a home pregnancy test and it’s positive, it’s time to make your first prenatal appointment. Your first appointment must be 10 weeks ago. This way your healthcare team can check baby for an estimated due date and plan your course of care over the next 9 months!
Whether you’re getting pregnant or just found out you’re pregnant, it’s always a good idea to take a prenatal vitamin and boost your immune system. Pink Stork’s Total Prenatal + DHA it has all the vitamins and nutrients you need to support a healthy pregnancy. And ours Prenatal probiotics supports a healthy gut and immune system to protect both you and your baby.
Bibliographical references:
America College of Obstetricians and Gynecologists. (2022). Methods for estimating the expiration date.
Flo. (2022). Pregnancy due date calculator.
Forbes Health. (2023) Due Date Calculator: When Is Your Baby Due?
