The moment you find out you’re pregnant, the questions start coming fast: How far along am I? When is my due date? What is going on in there right now?

Pregnancy calculators can help answer all of those questions, at least in part. There are a few different types, and they each do something a little different. Here’s a simple guide to what they are, how they work, and what they can and can’t tell you.

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Due Date Calculators

What they are

A due date calculator estimates the date your baby is expected to arrive, and it’s one of the first tools many newly pregnant people reach for. You enter the first day of your last menstrual period (LMP), and the calculator does the math.

How they work

Most due date calculators use what’s called Naegele’s Rule, a formula that adds 280 days (40 weeks) to the first day of your last period. It’s the same method your healthcare provider uses as a starting point.

Some calculators also let you enter a conception date, an IVF transfer date, or the results of an early ultrasound, which can give a more personalized result.

What they can tell you

A due date calculator gives you a solid estimated due date (EDD), a reference point for planning your prenatal care, understanding your trimesters, and getting a general sense of your pregnancy timeline. It’s a very useful starting place.

What they can’t tell you

Here’s the part that surprises a lot of people: a due date is an estimate, not a prediction. Only about 4% of babies are actually born on their calculated due date. Most arrive within a week or two on either side. It can help to think of your due date as the center point of a window, not a deadline.

Due date calculators are also less accurate for people with irregular cycles, since the standard formula assumes a 28-day cycle. If your cycles are longer, shorter, or unpredictable, your provider may adjust your due date after an early ultrasound, and that’s completely normal.

Try it

• American Pregnancy Association. An easy-to-use calculator from a reputable nonprofit. Accepts LMP, conception date, IVF transfer date, or ultrasound date. Also has a free helpline if you have questions.
• Cleveland Clinic. Medically reviewed and straightforward. A good option if you want something no-nonsense and medically reliable. 

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Conception Date Calculators

What they are

A conception date calculator works in the opposite direction from a due date calculator. Instead of starting with your last period and calculating forward, it starts with a known due date (or your LMP) and works backward to estimate when conception likely occurred. You may also hear this called a reverse due date calculator.

How they work

Since the average pregnancy is 266 days from conception to birth, the calculator subtracts 266 days from your due date to estimate when conception happened. It typically gives you a window of several days rather than a single date.

What they can tell you

A conception date calculator can give you a reasonable estimate of when conception occurred. That can be useful for understanding your pregnancy timeline, or simply satisfying the very natural curiosity about when your pregnancy began.

What they can’t tell you

Conception date calculators are estimates, and they can’t pinpoint an exact date. Sperm can survive in the body for up to five days after intercourse, and the egg is only viable for 12–24 hours after ovulation. That means conception could have occurred on any of several days within a window, even if you know roughly when you were ovulating.

Unless pregnancy was achieved through IVF (where the exact transfer date is known) a precise conception date simply can’t be determined. A good calculator will show you a range, not a single day.

Try it

• CalculatorSoup. Simple, no-frills, and honest about the uncertainty. It shows you a date range, not just a single day. No account required.

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Fetal Development Trackers

What they are

Fetal development trackers are week-by-week tools that show you what’s happening with your baby’s growth and development at each stage of pregnancy. They typically include size comparisons (your baby is now the size of a blueberry, a peach, a butternut squash…), information about which organs and systems are forming, and notes on what you might be feeling.

How they work

You enter your due date or the first day of your last period, and the tracker shows you where you are in your pregnancy week by week. Unlike due date calculators, development trackers are something you tend to return to throughout your pregnancy as each new week begins.

What they can tell you

Fetal development trackers offer a wonderful window into what’s happening inside your body at each stage of pregnancy. They can help you understand why you might be feeling certain things at certain times, give you a sense of the major developmental milestones, and make the whole experience feel a little more real, especially in those early weeks before you can feel movement.

What they can’t tell you

Development trackers show you what’s typical for a given week of pregnancy, not what’s specifically happening with your baby. Every pregnancy is different, and a tracker can’t tell you whether your baby is measuring on track, how they’re positioned, or anything about their individual development. That’s what your prenatal appointments are for.

If you have specific questions about your baby’s growth or development, your healthcare provider or doula is the right person to ask.

Try it

• WebMD Interactive Pregnancy Timeline.  A visual, week-by-week tool you can use right in your browser. No account or app download required. Medically reviewed.
• Cleveland Clinic Fetal Development Guide. A thorough, medically reviewed overview of fetal development by stage. Also browser-based, no account needed.

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A Note on Data Privacy

If you use browser-based calculators like the ones linked above, you enter a date, get an answer, and you’re done. There’s not much data being collected or stored.

Pregnancy and parenting apps are a different story. Many free pregnancy apps collect detailed information about your health and pregnancy and may share or sell that data to third parties, including advertisers. Unlike information you share with your healthcare provider, information entered into a pregnancy app is not protected by HIPAA.

This is worth knowing for a couple of reasons. One is simply that you may start receiving targeted ads and marketing materials based on your pregnancy data. The other is that targeted pregnancy marketing can continue even if you experience a pregnancy loss, which can be deeply painful.

If you decide to use a pregnancy tracking app, it’s worth taking a few minutes to read the privacy policy before you download. Look for whether they share data with third parties, and whether you can delete your data if you choose to.

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Questions? Your Doula Can Help

Pregnancy comes with a lot of information to sort through, and it’s completely normal to have questions along the way about your due date, your results, what something means, or what to expect next. A doula can support you throughout your pregnancy, as well as during your birth and afterward.

If you live in Linn, Benton, Lincoln, Lane, Marion, or Polk counties in Oregon and have Medicaid/OHP, you may qualify for free doula care through the Community Doula Program.

This post is for informational purposes only and does not constitute medical advice. Always seek the guidance of your physician or other qualified health professional with any questions you may have regarding your health or a medical condition.