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Calculate Embryonic Age vs Gestational Age: Pregnancy Math

How far along are you? Math can tell you, if your doctor hasn't already. Image by Decoded Pregnancy, all rights reserved

How far along are you? Math can tell you, if your doctor hasn’t already. Image by Decoded Pregnancy, all rights reserved

When you get pregnant, you’re going to hear the terms “embryonic age” and “gestational age” tossed around quite a lot, but how does your doctor or nurse calculate these ages – and what’s the difference between the two? You can use one number to find the other via a simple math equation – but if you want to know how far along you are, you may want to skip straight to Naegele’s Rule.

Gestational Age vs. Embryonic Age: How Old is Your Baby?

Before we get into the math, let’s talk about the terms.

Gestational Age

Your doctor probably uses the phrase “gestational age.” This is an estimate of the age of your fetus, calculated from the start of your last menstrual period, or LMP – yes, that’s before the baby was even conceived.

If Gestational Age starts before the baby is conceived, which affects accuracy, why do doctors use it? According to Medline Plus, gestational age provides a convenient and reliable starting point, especially for the woman who was trying to conceive a child. You were tracking your periods, right? So you know the start of your last menstrual cycle – even if you don’t know exactly when you conceived.

Embryonic Age

Embryologists, the biologists who study the development of an embryo, use embryonic age. If you know the exact date when the sperm fertilized the egg, you would use that date to “start the clock” for how developed the baby is. “fetal age” is another term that describes the exact age of the baby.

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Math to Convert Between Embryonic Age and Gestational Age

Generally, the date of conception is two weeks after the start of the last menstrual period. Therefore the math is very simple:

  • Gestational Age = Embryonic Age + 2 weeks,
  • Embryonic Age = Gestational Age – 2 weeks,

You don’t need a calculator to convert between gestational age and embryonic age, thank goodness.

In other words, if you had a fertility treatment such as receiving a fetus from in vitro fertilization, you were probably told the exact date of conception. You will track the embryonic age based on that date of conception, and add two weeks if you need to discuss gestational age with your doctor. Conversely, you can estimate your baby’s embryonic age by subtracting two weeks from your doctor’s estimate of his or her gestational age if you’d like to see exactly how much baby has grown.

Naegele’s Rule to Calculate Baby’s Age

‘Naegele’s rule’ was named after German Obstetrician Franz Naegele, even though he didn’t actually invent it. According to the Oxford Handbook of Clinical Examination and Practical Skills, botanist Harmanni Boerhaave actually developed the calculation, but Naegele publicized the rule for midwives in 1830.

Naegele’s rule calculates the EDD (Estimated Due Date) based on LMP (Last Menstrual Period), and therefore relates to gestational age. That calculation is:

EDD = LMP – 3 months + 1 year + 7 days

Let’s note that this is close to “EDD = LMP + 9 months.”

In other words, you will calculate your due date by subtracting three months from the first day of your last period, then adding one year and seven days.

How Far Along Are You?

You and your doctor may arrive at different dates, depending on whether you’re calculating gestational age via your last period, or embryonic age, based on your ovulation date, but only your baby (and possibly your ultrasound technician) knows for sure when he or she will be ready to be born.


Baskett, T., Nagele, F. Naegele’s rule: a reappraisal. (2000). British Journal of Obstetrics and Gynaecology. Accessed September 3, 2013.

Oxford University Press. Oxford Handbook of Clinical Examination and Practical Skills. (2007). Thomas, J., Monaghan, T. (eds).

Kaneshiro, Neil K, MD. Gestational Age. (2011). Medline Plus. Accessed September 3, 2013.

MacKenzie, Andrew P, MD; Stephenson, Courntey D, DO; Funai, Edmund F, MD. Prenatal assessment of gestational age. (2013). Up To Date. Accessed September 3, 2013.

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  1. hi , I had an US yesterday and the EDC is 19 weeks, my LMP is 04/18/2015 , I’m just trying to figure out when I have concieved this baby .


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