4 Major Reasons Why a Woman’s Pregnancy Due Date Changes

Is it normal for the due date to change? What causes the due date to change in pregnancy? Can my pregnancy due date be changed in the third trimester? Can my due date be wrong by 2 weeks?

Pregnancy due date changes are just one of the surprising things pregnancy can bring.

When your doctor gave you a due date, your mum-to-be mind surely started to anticipate the big day. You had a whole timeline planned – you estimated when to start buying baby clothes, figured out when to file for maternity leave or mentally scheduled renovations to transform your study into a nursery. You may have even considered the perfect start date for your nanny.

But what if your doctor takes it all back and says it’s not your due date after all? Suddenly, he’s changing your due date in your third trimester. What does that mean for you?

Due Date: When Will I Give Birth?

The moment we confirm that we’re pregnant and how far along we are in our pregnancy, our next question will be “When will I give birth?”

Knowing their pregnancy due date gives women ample time to prepare themselves physically, mentally, and even financially for giving birth. Another importance of knowing your estimated date of pregnancy is so you can track the progress of your pregnancy and more importantly, the development of your baby.

How to Calculate Pregnancy

So you want to know how to calculate pregnancy weeks? Well, it’s actually pretty straightforward! The most common way to determine how many weeks pregnant you are is by using a pregnancy calendar. This nifty tool helps you keep track of your pregnancy journey and estimate the number of weeks you’ve been carrying that little bundle of joy.

To calculate your pregnancy weeks using a calendar, you need to know the date of your last menstrual period (LMP). From there, count the number of weeks that have passed since that date. Each week usually starts on a Sunday and ends on a Saturday. So, for example, if your LMP was on a Sunday, you would complete one week of pregnancy on the following Sunday.

Now, if counting weeks doesn’t come naturally to you (don’t worry, it’s not everyone’s forte), there are also numerous pregnancy apps and online calculators that can do the math for you. Simply input your LMP, and these handy tools will calculate your due date and the number of weeks you’ve been pregnant.

Remember, though, that these calculations are just estimates. For a more accurate assessment of your pregnancy progress, it’s always best to consult with your healthcare provider. They can use other methods, like ultrasound measurements, to determine the exact number of weeks and ensure your baby is growing as expected.

Calculating the pregnancy due date

How does your doctor compute your due date?

The average length of a pregnancy is about 280 days or 40 weeks from the first day of your last menstrual period (LMP). The first day of your LMP is also considered the first day of your pregnancy, even if you most likely didn’t conceive until about two weeks later.

The 3-step period also known as Naegele’s rule is the most common way to calculate a woman’s pregnancy due date. It involves a simple calculation: add 7 days to the first day of your LMP and then subtract 3 months.

For example, if your LMP was September 12, 2022

1. Add 7 days to your LMP (September 19, 2022)

2. Subtract 3 months (June 19, 2022)

3. Change the year, if applicable (June 19, 2023)

In this example, your due date would be June 19, 2023.

However, if you don’t have any idea when your last menstrual period was, your doctor may order an ultrasound to find out your due date.

Usually, a doctor orders an ultrasound when there’s a history of irregular periods (in which case the Naegele’s rule may not apply), when the date of your LMP is uncertain, or when conception occurred despite the use of oral contraceptives. From there, she will give you an expected date of delivery by ultrasound.

How accurate is your due date by ultrasound?

Is it possible that the ultrasound says a different due date from Naegel’s rule?

According to the American College of Obstetricians and Gynecologists (ACOG), foetal measurements that were taken in the first-trimester ultrasound are the most accurate way of estimating a pregnant woman’s due date.

This is because the ultrasound allows your doctor to measure the crown-rump length (CRL), or the length of the foetus from one end to the other, giving you a more accurate description of how far along you are and how big your baby is.

What Causes My Due Date to Change?

So you’ve been through your first checkup with your OB-Gynaecologist, and you’ve had your first ultrasound where the doctor also calculated your due date. So what’s up with the doctor changing your due date in your third trimester?

While this doesn’t happen a lot, changing the pregnancy due date based on an ultrasound can still occur. One of the primary reasons a doctor changes your due date is when he sees that your baby is significantly smaller or larger than the average foetal size at your particular stage of pregnancy. Thus, it is possible for the due date wrong by 4 weeks or less, 2 weeks on average. 

Here are other possible reasons why your doctor would decide to change your initial due date.

1. Your pregnancy due date changes because you have irregular periods.

As mentioned earlier, to estimate your date of delivery, your doctor will ask when your last menstrual period was.

For women with irregular cycles, this can be tricky. An irregular period is when your cycle lasts for more than 28 days.

To calculate your due date, it’s important to remember that the time from ovulation to your next menstruation is about 14 days. So if your cycle is 33 days long, then ovulation most likely happened on day 18.

If, for instance, your last menstrual period (LMP) was October 1, just add 21 days (October 22) and then subtract 14 days to find out your LMP (October 8).

This is the reason why doctors order an ultrasound for pregnant women with a history of irregular periods. This way they can further determine what could be your due date. 

2. Your pregnancy due date changed because you had a second-trimester ultrasound.

A study by the American Journal of Obstetrics and Gynecology found that ultrasounds done in the first trimester – or specifically until 13 weeks of pregnancy – are more accurate in predicting your due date.

If you didn’t have a first-trimester ultrasound and your doctor only calculated your due date based on your LMP, they will refer to the second-trimester ultrasound for your new due date. So yes, it is possible that the ultrasound says a different pregnancy due date. 

 “If the date of your last period is several days off from our calculations, we give validity to the ultrasound,” says Christine Greves, an OB-Gynecologist at the Winnie Palmer Hospital for Women & Babies in Orlando, Florida.

If your pregnancy is the result of in vitro fertilization or IVF, your doctor will take into account the embryo’s age and the date of transfer to calculate your due date.

3. Your pregnancy due date changes because of an irregular fundal measurement.

During every doctor’s visit, your fundal measurement will be examined. Fundal measurement refers to the distance in centimetres from the pubic bone to the top of the uterus. This examination gives a clearer picture of how far along you are, based on the size of your uterus.

According to Mayo Clinic, the expectation is that after the 24th week of pregnancy, the fundal measurement will match how far along you are (the number of weeks), give or take 2 centimetres.

When you are carrying a bigger or smaller-than-average baby, your predicted due date can change. After your fundal measurement, your doctor can adjust your due date accordingly.

4. Your pregnancy due date changes because of abnormal Alpha-fetoprotein levels

Your alpha-fetoprotein (AFP) levels, which is a protein produced by your growing baby’s liver and yolk sac, are usually measured between week 14 and week 22.

AFP blood tests in pregnancy can help detect if there are problems in a baby’s neural tube, or what will become the brain and spinal cord.

In some cases, a positive AFP result means there is a chance your unborn child has a genetic abnormality.

If your AFP level is high, it can mean your predicted due date is inaccurate.

In some cases, it means there is a chance your unborn child has a genetic abnormality.

If your AFP level is high, it can mean your predicted pregnancy due date is inaccurate. You may be further along than you thought because your level keeps going up throughout your pregnancy. Your doctor might need to adjust your due date in that case.

Could My Due Date Be Wrong by a Month?

While getting your expected due date off by a month is rare, it is not impossible. As your pregnancy progresses, the accuracy of the expected date of delivery by ultrasound decreases.

Between your 18th to 28th week, the margin of error in your ultrasound telling your due date increases to an average of 2 weeks. After the 28th week, the ultrasound may be off by over 3 weeks in predicting the pregnancy due date.

So Your Due Date Has Changed – What Now?

Your due date is a significant milestone during pregnancy, but what happens when it changes? Don’t worry, you’re not alone! Many expectant parents experience a shift in their due dates, which can initially cause confusion and anxiety.

However, understanding the nature of due dates and staying informed about your baby’s development can help you navigate this change with confidence. Here’s a handy guide to help you embrace the unexpected and prepare for your little one’s arrival.

• Due Dates Are Estimates, Not Exact Science

Keep in mind that due dates are not set in stone. According to a 2013 study, only 5 per cent of women give birth on their exact due dates. Your due date is merely a guide, giving you a general timeline for when your baby might arrive. So, don’t stress if it changes—it’s more common than you might think!

• Embrace Being Prepared for the Unexpected

As a parent-to-be, being prepared for unexpected changes is an invaluable skill to develop. Use the change in due date as an opportunity to adapt and adjust your plans accordingly. Flexibility is key, and remember, no matter when your little one arrives, you’ll be ready to provide them with love and care.

• Track Your Pregnancy and Baby’s Development

Besides routine checkups, actively monitoring your pregnancy and your baby’s development can help you stay connected and reassured. Use pregnancy tracking apps that provide week-by-week updates on your baby’s growth, provide tips, and even let you record your baby’s kicks during the third trimester. 

• Consult Your Doctor

If you have any questions or concerns about the change in your due date, don’t hesitate to reach out to your healthcare provider. They are the best source of information and can address any uncertainties you may have. Your doctor will guide you through the process and ensure that you and your baby are receiving the appropriate care.

Remember, pregnancy is a unique journey, and each baby arrives in their own time. Stay positive, stay informed, and embrace the changes that come your way. Your little one will be in your arms before you know it. It’s true that you can’t plan every little thing about parenting. But you can prepare yourself for whenever your baby is ready to come out into the world!

Additional information by Camille Eusebio

Here at theAsianparent Singapore, it’s important for us to give information that is correct, significant, and timely. But this doesn’t serve as an alternative for medical advice or medical treatment. theAsianparent Singapore is not responsible for those that would choose to drink medicines based on information from our website. If you have any doubts, we recommend consulting your doctor for clearer information.