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 confirmed 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.

Calculating the pregnancy due date

So 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 three-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 seven days to the first day of your LMP and then subtract three months.

For example, if your LMP was Sept 12, 2022

Add 7 days to your LMP (Sept 19, 2022)

Subtract 3 months (June 19, 2022)

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 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 to be wrong by four weeks or less, two 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, a 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 two 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 two weeks. After the 28th week, the ultrasound may be off by over three weeks in predicting the pregnancy due date.

So your due date has changed - what now?

For most of your pregnancy, you have referred to your due date as the tipping point or culmination of everything that you have been preparing for. So a change in due date might leave you in a rut, dampen your plans and cause confusion when it comes to your baby's health and development.

But remember, nobody really knows the exact date and time they'll give birth. Your pregnancy due date is merely a guide or a timeline for giving birth. In fact, a 2013 study revealed that just five per cent of women give birth exactly on their due dates.

Due date miscalculations can cause anxiety for most parents, but the important thing is to be prepared for the unexpected. This is, after all, a useful skill that parents can start honing even before their baby comes.

Aside from the routine checkups, it's important that you also track your pregnancy and your baby's development. Be informed of your child's progress week by week. Monitor your baby's kicks starting the third trimester. There are several mobile apps that you can use to better monitor your pregnancy.

If you have any questions regarding your due date and when you'll give birth, don't hesitate to consult your gynaecologist.

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!

This article was first published in theAsianparent.