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Gestational Age: How Do You Count Pregnancy Weeks?

Learn about gestational age — the number of days or weeks since the start of a person’s last menstrual period — and why it’s used to track pregnancy.

What is gestational age?

Gestational age is the number of days or weeks that have passed since the first day of a pregnant person's last menstrual period. Knowing your gestational age while pregnant allows you and your health care provider to estimate your baby’s due date. 

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

How to understand gestational age

LMP refers to the first day of your last menstrual period. Health care providers measure pregnancies in weeks starting from the first day of the LMP. If your period is regular and lasts 28 days, and if ovulation generally happens on day 14 of your cycle, then conception probably took place about two weeks after the LMP. For gestational age counting, these two weeks are added to a pregnancy as a simpler method than trying to track from ovulation or fertilization.

Nevertheless, this estimation is not always accurate. It doesn't take into account factors like changes in the menstrual cycle, variations in ovulation, and imperfect recall by patients. 

A due date calendar or calculator can help pregnant people 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). 

Different calculators can help determine your due date based on ovulation, date of conception, and ultrasound data. Specific calculators can predict the due dates of people who are using assisted reproductive technologies and embryo transfer.

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The conception date refers to when a baby was conceived. Since sperm can live inside the vaginal canal for three to five days, conception can take place up to five days after sexual intercourse. Fertilization depends on ovulation and can occur within 12-24 hours from the release of the egg (ovulation).

The delivery date is the day when a person gives birth. A person’s delivery date may differ from their due date for a variety of reasons. A person can go into labor earlier than their due date, later than their 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 retrieve live images from inside your body. During the test, waves pass through the body, come into contact with tissues, and reflect off them. These reflected signals enter the machine and are converted into an image. All of these steps happen in real-time.

For a pregnancy, health care providers use ultrasounds, also known as a sonogram, to measure the baby in the uterus. While looking at a live picture, health care providers and technicians can determine a baby's sex, check on their health, rule out genetic issues, measure their growth, and usually determine their fetal age. An ultrasound also helps with some procedures like amniocentesis, in which a sample of amniotic fluid and cells is taken with a needle.

Gestational age

Gestation consists of the time between the conception date and birth — or how long a person is pregnant in weeks. Gestational age is measured from the pregnant person’s LMP to the current date in weeks. 

Based on your gestational age, you can glean accurate information about your pregnancy. An accurate gestational age is necessary for planning obstetric care and preventing potential complications. The milestones of a baby’s growth are measured in fetal age but should generally align with gestation.

IVF calculating

To use an IVF pregnancy calculator, we 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 a person’s uterus. Since IVF is a different way of conceiving than naturally (having sexual intercourse), calculating your gestational age, due date, and fetal age after IVF is a bit different. 

For people who conceived with IVF, their gestational age and due date can be based on:

  • egg extraction
  • insemination
  • blastocyst transfer

An IVF pregnancy calculator is highly accurate, as the actual conception happens at a confirmed time outside of the body. There are several formulas that IVF pregnancy calculators can use, depending on what type of IVF you had.

  • If fresh eggs are used, add 266 days to the date of egg extraction or fertilization.
  • If three-day-old frozen embryos are used, add 263 days to the embryo transfer date.
  • If the frozen embryo is five days old, add 261 days to the embryo transfer date.

Doctors sometimes use the term appropriate for gestational age (AGA), which means that a baby’s gestational age findings match their calendar age at birth. Full-term babies born AGA usually weigh from 5.5 to 8.75 pounds.

If a full-term baby is born above or below this range, they are large for gestational age (LGA) or small for gestational age (SGA). They may experience some health issues later in life.

In addition to weight, health care providers also measure gestational age after birth by checking your baby's skin and hair, head and chest circumference, length, muscle tone, posture, and vital signs.

Gestational age vs fetal age

Fetal age is separate from gestational age. While the gestational age measures how far a pregnancy has progressed in weeks, fetal age is the growing baby’s actual age. 

Fetal age is counted from the moment of conception. Thus, it is 14 days less than the gestational age, assuming a regular menstrual cycle of 28 days. Health care providers generally measure fetal age through ultrasounds. An ultrasound lets them measure the size of your baby's head, abdomen, and thigh bone, which together indicate fetal age.

Estimated due date

In order to estimate your baby’s due date, physicians look at your LMP and first ultrasound examination. They’ll generally add 280 days (i.e., 40 weeks) to the date of your LMP, compare it with the first ultrasound, and find the estimated due date.

Drawbacks of using gestational age

Due date predictions are not always accurate; to some degree, they’re an assumption. Some pregnant people even report their health care providers changed their due dates in the third trimester. In most cases, the due date based on a first-trimester ultrasound is considered the most accurate.

If the first ultrasound is performed after 22 weeks of gestation, it won’t be quite as accurate in estimating the due date. In this case, the pregnancy will not be considered optimally dated.

In conclusion 

To get the most accurate information about your pregnancy, due date, conception date, or IVF pregnancy calendar, 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. This can help you prepare for birth and the newborn season, encouraging your emotional and physical well-being.

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MacKenzie, Andrew P, et al. “Prenatal Assessment of Gestational Age, Date of Delivery, and Fetal Weight.” UpToDate, www.uptodate.com/contents/prenatal-assessment-of-gestational-age-date-of-delivery-and-fetal-weight.