You can find out your due date almost as soon as you get a positive pregnancy test.
It’s so exciting to find out when you’ll meet your baby. But it’s also confusing to see different dates and clinical abbreviations on your chart. For example, what does AUA mean on a scan?
Many women don’t understand the terminology used on their scan results. They wonder why they’ve been given different dates.
Let’s take a look at estimated due dates and the different ways to calculate them.
#1: Why is it important to have an estimated due date?
It’s rare these days for a pregnant woman not to know, to the day, how many weeks pregnant she is and the date her baby is due.
It’s not just because she’s excited about her baby’s birth. From the moment she confirms her pregnancy, all her care is focused on the progress of her baby’s development.
Your care provider can put into place various interventions and management if your baby’s development doesn’t meet the ‘expected’ averages.
These averages are based on your baby’s expected development each week of pregnancy. Babies have a lot of growing and developing to do in 9 months.
Decades of research have provided us with the amazing knowledge of what fetal development looks like. This is how we know, for example, that by week 10 babies have all their organs, even if they’re not all working yet.
Accurate pregnancy dating is key to making sure you and your baby are on track for a healthy pregnancy and birth.
#2: Are estimated due dates always right?
We’d love to tell you estimated due dates are always right but the truth is they rarely are!
It’s estimated that only 3-5% of babies are actually born on their due date. The rest are born in the two weeks before or after that highly anticipated day.
Read Estimated Due Dates And The Myth Of The 40 Week Pregnancy for more information.
#3: What is the last menstrual period (LMP) date?
The average pregnancy lasts around 40 weeks; technically this is 38 weeks from when conception occurred.
Based on this average, the easiest way to estimate your due date is to count 40 weeks (280 days) from the first day of your last menstrual period.
Your estimated due date is then settled on as the day you can expect your baby to be born.
Check out BellyBelly’s Due Date Calculater to find out your estimated due date.
#4: What is gestational age (GA) on an ultrasound?
It’s now very common for pregnant women to have a dating or first trimester ultrasound.
When you look at your ultrasound print out, you’ll usually see the letters GA, meaning gestational age.
Gestational age means how far along your pregnancy is. It’s based on your last menstrual cycle and assumes a 40 week pregnancy from that date.
#5: What does AUA mean on a scan result?
What does AUA mean on a scan? And why do you see a due date that’s different from your gestational age due date?
AUA stands for actual ultrasound age. This refers to the age your baby is from conception, based on the baby’s measurements.
Remember, the gestational age is calculated from the first day of your last period and is how many weeks pregnant you are. But conception doesn’t happen until around the middle of the month.
By the time you’re 4 weeks pregnant, your baby has been developing for only 2 weeks.
By the time you have a first trimester scan, there could be a difference in dates, depending on the variations of your menstrual cycle length and timing of conception.
#6: Is ultrasound more accurate for estimating the due date?
First trimester dating is the most accurate way to estimate your baby’s due date.
This is because the scan measures the size of your baby. This gives the most accurate estimation of the baby’s age, based on development.
The earlier a dating ultrasound is done, the more accurate the estimated due date is.
Usually, ultrasounds in the first trimester (up to 12 weeks) are within 3-5 days accurate.
For the most accurate results, the best timing for the ultrasound is between 8 and 11 weeks gestation. During this time your baby is growing very rapidly so the size differences are more obvious.
As babies get bigger, they start to express their individual growth rate. As time goes on, their size correlates less and less with their actual age.
This means ultrasounds after 22 weeks gestation aren’t used to estimate due dates because the baby’s size doesn’t reflect actual age very well. In fact, even average babies can differ by up to 2-3 weeks of growth.
#7: Why is my ultrasound due date different?
If you’ve had an early pregnancy scan and the dates don’t match the estimated due date you already had, you might feel a little confused and even worried.
The scan gives a more accurate estimate of when your baby is due, based on fetal development. If this date is within 5 days of the EDD from your last menstrual period then your care provider might stick with that original date.
The EDD from the dating scan is used if your last menstrual period is unknown or unreliable, or if the date differs by more than 5 days from your LMP dating.
If there are any concerns about your baby’s development then it’s likely you’ll be referred for a follow-up scan in a few weeks.
If you have more than one ultrasound during pregnancy, each with a different date, the earliest ultrasound estimated date should be used, as it’s the most accurate.
But remember, babies are born when they’re ready, not because a date has been marked on your calendar.
Your baby has plenty of developing to do before it’s time to meet the world outside the womb.
Be sure to read What Causes Labour To Start to find out what needs to happen before your baby is ready to be born.