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Having an estimate for when you’re going to deliver your baby is very important for growing families. Not only does it help you get everything prepared before you take maternity leave, but it can also ensure that you receive optimal prenatal care.
With that said it can be confusing to determine your baby’s age, especially considering healthcare professionals will use terms like gestational age and fetal age interchangeably.
In this guide, we’ll show you the differences between gestational and fetal age and how to calculate both. We’ll also help you determine your due date based on those calculations.
What is Gestational Age?
Gestational age describes how far along a pregnancy is and is measured in weeks. To determine the gestational age, you would count how many weeks it’s been since the first day of your last period. If your last period started four weeks ago, then the gestational age is four weeks.
A full-length pregnancy ranges from 39 to 40 weeks. However, any baby born before 37 weeks is considered premature, and any baby born after 42 weeks is considered postmature.
It’s important to note that gestational age is considered more accurate than fetal age, but it can’t always determine the exact age of your developing baby. Variations in ovulation, changes in menstrual cycles, and poor menstrual cycle recall can impact your gestational age calculation.
However, if you’re interested in getting the most accurate gestational age and due date, an ultrasound scan is the best method for this, especially when done during the first trimester.
While fetal age and gestational age are often used interchangeably, they aren’t the same thing.
What is Fetal Age?
According to the experts at Flo, fetal age represents the “growing baby’s actual age.” As the fetal age is counted from the moment of conception, it’s 14 days less than the gestational age, but only if you have a regular 28-day menstrual cycle. Fetal age is less precise than gestational age, as it’s difficult or sometimes impossible to know the exact moment conception occurs.
As the fetal age is 14 days less than the gestational age, you could determine fetal age by subtracting two weeks from your gestational age.
How Do You Determine Gestational or Fetal Age?
The easiest way to determine gestational age is by remembering or tracking your menstrual cycle before conception. From there, you can determine fetal age by subtracting 14 days from your gestational age, as stated. But what if you haven’t been tracking your menstrual cycle? Or, what if you want a more accurate way to determine your baby’s age before and after birth?
If you conceived naturally (sexual intercourse), here’s how to determine a baby’s age:
- Before Birth: Ultrasound is the most accurate way to determine gestational or fetal age before birth. The tech will take measurements of your baby’s body parts (head, thigh bone, and abdomen) to check how they’re growing. Ultrasound techs will have certain expectations of growth depending on the baby’s age, making it easier to determine how old they are.
- After Birth: Doctors will determine gestational or fetal age after birth by the baby’s weight, head circumference, reflexes, vital signs, length, posture, muscle tone, and skin and hair condition. Determining a baby’s gestational age after birth is only necessary if the mother did not realize they were pregnant, didn’t have access to prenatal care, or declined prenatal care or scans.
If you’ve conceived by in vitro fertilization (IVF), then an ultrasound and after-birth measurements are still effective for determining gestational and fetal age.
However, instead of using the day of your last period to roughly calculate gestational age, you would use the date of insemination or embryo transfer.
If the baby’s age matches the calendar age, they’re determined to be appropriate for gestational age (AGA). Whether conceived naturally or via IVF, babies born of the AGA will usually weigh between 5.5 lbs (2.5 kg) and 8.75 lbs (4 kg) at birth.
How Do You Determine Your Due Date?
To determine your due date, you’ll use many of the same calculations already mentioned in this article, except you’ll add specific numbers to estimate when you’ll likely give birth. Keep in mind that it’s nearly impossible to determine an accurate due date, as we can’t tell the future.
With that said, here’s how to estimate your due date if you conceived naturally:
- Calculate From the Date of Your Last Menstrual Period (LMP): From the date of your LMP, add 40 weeks, as that’s the middle number between the average pregnancy range.
- Calculate From the Day You Conceived: From the date you conceived, add 38 weeks. As stated, it’s difficult to determine the date of conception (or fetal age), but you can track your ovulation to estimate when you likely conceived using the basal body temperature method or by tracking ovulation symptoms, like cervical mucus or pain.
If you used egg collection and IVF, you would calculate your due date as followed:
- For fresh eggs, you would add 266 days to the date of fertilization or egg extraction.
- If the embryo was transferred after a few days, you would subtract the number of days after the eggs were fertilized or retrieved. For example, if the egg was transferred 5 days after it was retrieved, then you would subtract 5 from 266 and get a due date of 261 days.
Ask your provider to know exactly when your egg was retrieved to determine a viable due date.
While these calculations won’t determine the exact age of your baby or your due date, they can give you some indication of how far along you are. However, the more information you have on your conception date, the more accurate you’ll be when determining your baby’s growth cycle.
It also helps to use the most accurate methods to determine gestational age. This means getting an ultrasound done by an experienced tech during the first trimester.