Many health care professionals will begin to estimate how far along you are in your pregnancy by asking you when the first day of your last menstrual period (LMP) was. But the development of your baby does not begin until conception – which is after your last period.
Are you confused? Let me explain…
Your pregnancy has two ages, gestational age and fetal age. Both are measured in weeks but they will be different numbers. Your gestational age is the age of the pregnancy from the first day of your last normal menstrual period. Your fetal age is the actual age of your growing baby from the day you conceived.
Health care providers use gestational age when dating a pregnancy. It is very difficult to determine an accurate date of conception, so your health care provider may estimate when you conceived based on the first day of your last period. This is your gestational age.
While your provider will use your LMP to initially date your pregnancy, according to the American Congress of Obstetrics and Gynecologists (ACOG), an ultrasound measurement of your baby in the first trimester is the most accurate method of confirming your gestational age. Keep in mind, after a first trimester ultrasound, your due date may be adjusted.
Why is it important to be aware of both?
We often hear from pregnant women that their baby’s development is not matching up with how far along they are in their pregnancy. For example, one question we received, the woman was 12 weeks pregnant but her baby was measuring 10 weeks along. It is important to confirm with your provider that your gestational age is 12 weeks and your fetal age is 10 weeks – which means your pregnancy is on track. (Keep in mind that there may be other reasons why a baby is not developing on a typical schedule. If you are concerned, speak with your prenatal provider.)
Not sure how far along you are? Our due date calculator, will date your pregnancy based on gestational age.
Still have questions? Text or email us at AskUs@marchofdimes.org.