Great Pregnancy Due Date Calculator

Congratulations on your pregnancy news!

One of the first things people want to know in early pregnancy is how long they have to prepare. BellyBelly’s pregnancy due date calculator will give you an estimated due date (EDD) of birth to mark on your calendar.

You don’t need to know the date of conception; all you need to know is the first day of your last menstrual period, and BellyBelly’s pregnancy due date calculator will do the rest.

How do I calculate when I am pregnant?

The pregnancy due date calculator is an easy way to figure out when your baby is due. You can use your due date to start planning the next stage of your life. You might want to start thinking about maternity leave, creating or decorating the nursery, and when to announce your pregnancy. Although the calculator isn’t exact, it can help you sketch out a rough pregnancy timeline to see how your life might look for the next few months.

Remember, the estimated date is what it says – just an estimate; it’s not a delivery date. You can’t track the parcel or expect it to arrive on time. Babies come when they’re ready. Rather than seeing the due date as a concrete date, see it as a place marker for when your baby might arrive, when the time is right.

Once you have your due date worked out, be sure to sign up for BellyBelly’s Pregnancy Week By Week emails – they’re awesome!

What is a pregnancy due date calculator?

A pregnancy due date calculator is an easy way to figure out when your baby is due. It’s not an exact science but it will give you an estimated date of when to expect your baby to arrive.

The due date calculator won’t give you the exact date of your baby’s birthday as it is based on a number of estimations, such as your last period and your cycle length. Instead, it provides an estimated due date while you wait for your dating ultrasound scan.

How does a pregnancy due date calculator work?

Usually, a due date calculator works out approximately 40 weeks (or 280 days) since the first day of your last menstrual period. The BellyBelly pregnancy due date calculator is slightly fancier because it takes into account your average cycle length (not just the day of your last menstrual period) and, if you know it, when you entered your luteal phase. This information increases the accuracy of the calculations, providing better individual results to help you prepare for your new arrival.

How to use a pregnancy due date calculator

To use the calculator, you’ll need to know the first day of your last menstrual period, your average cycle length, and when you entered your luteal phase. Simply input the information, and the calculator will crunch the numbers to provide you with an estimated date for your baby’s birth.

When to take pregnancy test calculator?

Use the calculator during your first trimester to figure out when you might meet your little one.

The BellyBelly pregnancy due date calculator is easy to use, so you can find out your baby’s due date as soon as possible. Your first ultrasound scan is the most reliable way to determine your estimated due date, so most pregnancies have a varying due date until the sonographer measures your baby at the dating scan.

In the meantime, however, the pregnancy due date calculator can provide you with an estimate so you can start to prepare and count down to the birth of your baby.

How accurate is a pregnancy due date calculator?

When using the calculator, it’s essential to bear in mind that the estimated date is just an estimate. It’s unlikely your baby will actually arrive on that date, although it should give you a rough idea of when your baby is due.

The accuracy of a pregnancy due date calculator is limited by the information you have at hand, due to various reasons, including:

• The accuracy of the information you put in
  The formula uses the day of your last menstrual period to calculate your results. If you don’t know the exact date of your last period, you’re unlikely to get accurate results
• The fact that pregnancy doesn’t last exactly 40 weeks
  Babies come when they’re ready. Your baby doesn’t have a tiny countdown calendar hung up on the wall of your uterus. Some babies are born at 37 weeks; others are born at 42 weeks. A study found that even when the date of ovulation occurring was known, the length of pregnancy varied by 37 days
• The formula assumes you have a 28-day menstrual period or cycle and ovulate on day 14
  The due date calculator uses the same formula as your midwife does when she gives you an early estimated due date at your booking-in appointment. According to a 2021 article in the Australian and New Zealand Journal of Obstetrics and Gynaecology, the formula was devised in the 1800s when very little was known about the complex nature of the female reproductive system.
  The formula assumes you have a 28-day menstrual period or cycle, which many women don’t have. If you have irregular periods or an irregular cycle length and can’t determine when your last period was, the formula won’t take your unique cycle into account.

How are pregnancy weeks calculated?

Many women are confused to discover they are four weeks pregnant by the time they get a positive pregnancy result. Pregnancy is technically counted from the first day of your last menstrual period, so for weeks one and two of pregnancy, you’re not actually pregnant yet (confusing, right?).

Conception usually occurs around week 2 of pregnancy and you’re already said to be four weeks pregnant by the time you have had a missed period, pee on a stick, and see a positive pregnancy test.

Understanding and tracking your basal body temperature (BBT) may also help you identify your fertility window, as it rises slightly after ovulation.

For more information about this, see BellyBelly’s article Best Implantation Calculator For Pregnancy.

Due date calculator week by week

If you have longer or shorter cycle lengths, BellyBelly’s pregnancy calculator can take that into account. All you need to do is enter the first day of your last menstrual period (LMP), then make any cycle adjustments (optional).

You’ll notice that making slight adjustments to the length of your cycle and luteal phase can affect the due date. You’ll be able to discover the approximate date of your baby’s arrival, but keep fluctuations in mind. Standard calculators or charts use averages, the same method a doctor or midwife would use. But remember, our bodies aren’t based on averages.

Once you have your due date worked out, be sure to sign up for BellyBelly’s Pregnancy Week By Week emails.

It’s like having a pregnancy calendar and journal all in one. You’ll know what to expect and what’s ‘normal’ for every week of your pregnancy. It gives you a good glimpse of your pregnancy milestones. Be sure to speak to your doctor or health care provider so you can start getting some quality prenatal care with your choice of maternity care.

Menstrual cycle length

Most of these pregnancy due date tools are ‘one size fits all’. They take the first day of your last menstrual period (LMP) as day zero and assume your cycle is regular and lasts 28 days. Ovulation is assumed to have happened in the middle of your cycle and that date is usually considered to be your conception date.

The date of conception, however, isn’t necessarily the same as the date you ovulate. An egg can live for up to 24 hours, so conception can happen a day after the egg is released. You might find ovulation occurring on a different day, and your last period might be between 23 and 31 days in length.

If you don’t know when the first day of your last period was, and if you don’t track your fertile days, you might not be able to tell when you conceived, which could make determining the gestational age harder.

Our article Menstrual Cycle – 4 Different Stages And Phases is packed with information about the menstrual cycle.

Using ultrasound to calculate due date

Having ultrasounds is a common practice during pregnancy, so your dating scan will not be your first or last ultrasound. The best way to determine your due date is at your first ultrasound date scan. Although this, too, isn’t 100% accurate, a 2022 study published in AJOG Global Reports found that ultrasounds provide quite an accurate way to determine gestational age.

Dating scans are generally carried out towards the end of the first trimester. According to Pregnancy Dating, ultrasounds carried out after 14 weeks are less accurate for determining gestational age.

How is gestational age calculated at an ultrasound scan?

Your ultrasound date is generally between weeks 11 and 14 of pregnancy, because this is the best time to determine gestational age accurately. At this early ultrasound, the sonographer will measure your baby from crown to rump and use this measurement to calculate your baby’s gestational age and due date.

Other BellyBelly due date calculators to check out:

Now that you’ve stumbled across this helpful pregnancy due date calculator, you’re probably keen to know whether BellyBelly has any other nifty calculators hidden away. Lucky for you, we do. Check out our pregnancy tools page for the following calculators:

Due date calculator from conception

Sometimes you are sure of the date of conception, as it might have been the only time you had sex. In that case, a conception due date calculator will count this date as day 14, the last day of week 2.

If you’re sure about this, the numbers might not add up with your last period date, but you know your body, and that knowledge can give you a clearer idea of when your baby’s due date will be.

IVF due date calculator

For those who have achieved a pregnancy thanks to in-vitro fertilization (IVF), the egg retrieval, sperm collection, embryo age and fertilisation would have happened earlier.

See BellyBelly’s IVF Due Date Calculator.

When the date of the embryo transfer is known, that date should be taken as day 14, just between weeks 2 and 3.

If you conceived by artificial insemination, that date is also taken as day 14.

Reverse due date calculator

The Reverse Due Date Calculator uses a method that’s the exact opposite of a due date calculator.

It works when you know your pregnancy due date and want to figure out when conception happened. The reverse due date calculator is helpful if you want to know when you conceived, but can’t remember your last menstrual period date.

Pregnancy weight gain calculator

BellyBelly’s Pregnancy Weight Gain Calculator makes it easy to track your weight gain during pregnancy. This might help manage your physical fitness and keep you within a healthy weight gain range while you’re pregnant.

We hope that these tools and features are useful and we wish you all the best for an amazing and healthy pregnancy!