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Gestational Age: How Do You Count Pregnancy Weeks?

Learn about gestational age – the number of days and weeks that have passed since the first day of the mother's last menstrual period – and why it is used to track pregnancy.

What is gestational age?

Knowing and understanding your gestational age while pregnant is crucial to counting the pregnancy weeks and making an accurate guess about your baby’s due date. 

Gestational age describes how far along you are in your pregnancy. It’s measured from the first day of your last menstrual period (LMP) to the current date, in weeks. Usually, pregnancies last anywhere from 38 to 42 weeks and babies born before the 37-week marker are considered premature.

Terminology

In order to understand the gestational age, there are a few keywords you need to know.

The last menstrual period or LMP refers to the first day of your last normal menstrual period. We measure pregnancies in weeks starting from the first day of the LMP, so if your period is regular and ovulation generally happens on Day 14 of your cycle, then we can make the educated guess that conception took place about two weeks after the LMP.

Due date calendar is something pregnant women use to determine their baby’s due date. To calculate your due date, add 280 days (40 weeks) to the first day of your LMP (if you have a 28-day cycle). 

Conception date or date of conception refers to when a baby was conceived. Since sperm can live inside the vaginal canal for three to five days, conception can theoretically take place up to five days after sexual intercourse.

The delivery date is the actual date of when a woman gives birth. The delivery date might be different from the due date for a variety of reasons. A woman can go into labor earlier than the due date, later than the due date, require an emergency Cesarean or have to be induced. All of these factors can cause the delivery date to be different than the due date.

An ultrasound is a medical test that uses high-frequency sound waves to grab live images from the inside of your body. In pregnancy, we use ultrasounds—also known as a sonogram—to see and measure the baby while it is inside of a woman’s uterus. While looking at a live picture, doctors and technicians can determine a baby's sex, check on their health, measure their development, and usually determine their fetal age.

To understand what an IVF pregnancy calculator is, you first need to know what IVF is. In vitro fertilization (IVF) is a medical procedure in which an egg is fertilized by sperm in a test tube outside of the body, then implanted into the body where it can grow into an infant. Since IVF is a different way of conceiving than naturally (having sexual intercourse), calculating gestational age, due date, and fetal age with an IVF baby is a bit different. An IVF pregnancy calculator is very accurate, as the actual conception happens outside of the body, where it can be confirmed. There are several formulas IVF pregnancy calculators can use, depending on what type of IVF you had.

Gestational age

The gestation is the time between the conception date and birth, or how long a woman is pregnant in weeks. Gestational age is measured from the LMP to the current date in weeks. Most commonly, pregnancies last anywhere from 38 to 42 weeks. If a baby is born before 37 weeks, she is considered a premature (or preemie) baby.

Gestational age is crucial to getting accurate information about your pregnancy, as it is used to predict your baby’s due date. The milestones of a baby’s growth are measured in fetal age, but should generally align with where you are in your gestation.

Appropriate for Gestational Age (or AGA) means that a baby’s gestational age findings match the calendar age after birth. In AGA babies, there is less likelihood of complications and lower rates of death. Full-term babies born AGA usually weigh anywhere from 5.5 lbs to 8.75 pounds.

Doctors can also measure gestational age after birth by checking your baby's condition of the skin and hair, head circumference, length, muscle tone, posture, vital signs, and weight.

Gestational age vs fetal age

Fetal age is a separate unit of measurement from the gestational age. While the gestational age measures how far along a pregnancy is in weeks, fetal age is the actual age of the growing baby. Doctors generally measure fetal age through ultrasound. An ultrasound lets the doctor measure the size of your baby's head, abdomen, and thigh bone, which all inform the infant's fetal age.

Estimated due date

In order to estimate your baby’s due date, physicians look at your LMP. To calculate the due date, doctors generally add 280 days (which is 40 weeks) to the first day of your LMP.

Your last period and ovulation account for the first two weeks of pregnancy. Remember though that the due date is separate from the delivery date, as a due date is just a well-informed guess.

Why gestational age is important 

The more accurately doctors can guess your gestational age, the more accurately they can predict your due date. Gestational age is also important for knowing how far along your pregnancy is and therefore understanding the milestones.

Without finding the true gestational age, predictions as to which trimester you are in, and which milestones should be happening each week (such as baby growth, whether or not you can feel your baby kicking, etc.), would be inaccurate.

Why doctors use gestational age to calculate the due date

Gestational age is crucial to calculating your due date because your body is cyclical. The last week of your period and ovulation account for two full weeks of your menstrual cycle. Knowing that LMP gives doctors a clear jumping off point for predicting your due date. Just add 40 weeks from that LMP and your doctor should have a pretty accurate idea of when your baby will come. 

Drawbacks of using gestational age

Due date predictions are not always accurate. Some pregnant women even report their doctors changing their due dates come the third trimester. In most cases, the due date chosen in the first trimester is the most accurate.

One reason why a due date might change in the third trimester is that ultrasounds become less accurate further along in a pregnancy. If your physician changes your due date thanks to an ultrasound done in the third trimester, stick to your first trimester due date as the information from that first ultrasound is likely more accurate.

Gestational age is not always accurate, however. Since your due date is often based off your LMP, that puts it about two weeks off from the actual conception date. If a doctor does not add those two weeks before conception, your gestational age might have been miscalculated.

In conclusion 

If you want the most accurate information about your pregnancy, due date, conception date, due date calendar, or IVF pregnancy calendar, then calculating an accurate gestational age is crucial.

The more accurate information you have about how far along you are in your pregnancy, the more information you'll know about your baby's growth milestones and your own pregnancy milestones.