Gestational Age Calculator - How Do You Count Pregnancy Weeks?

Gestational Age Calculator - How Do You Count Pregnancy Weeks?

Gestational Age Calculator

  • Gestational Age
  • 0 years 0 months 0 days/
  • 0 months 0 days/
  • 0 weeks 0 days/
  • 0 days/
  • 0 hours/
  • 0 minutes/
  • 0 seconds/

Gestational Age Calculator

  • The Gestational Age Calculator is used to quickly determine the gestational age of your baby with only one piece of information.
  • This calculator helps you estimate date of delivery (EDD) ("due date") based on the timing of your last menstrual period (LMP).
  • Gestational age estimates how far along you are in your pregnancy. Your healthcare provider uses the gestational age in weeks (rather than months) to plan your care during pregnancy.


Steps To Calculate Gestational Age Calculator

  1. Enter the Pregnancy Estimated due date
  2. Enter Current Date
  3. Check Gestational Age


Gestational Age (GA) Formula

Assessing gestational age by the due date - this is the method this calculator uses and runs on the following calculations:


Date of LMP = EDD - 280 days

GA = Current date - Date of LMP



  • EDD is the estimated due date.
  • LMP is the last menstrual period.
  • GA is the gestational age.

What is Gestational Age

  • Gestational age (GA) is a common term used to describe where you are in your pregnancy or the age of a pregnancy.
  • In other words, this is the amount of time that the baby has been in the mother’s belly.
  • It is measured in weeks, usually by calculating the time difference between the first day of the woman’s last menstrual cycle and the current date.
  • According to this method, a baby that has just been conceived (approximately two weeks after the last period) is already two weeks old in gestational age.
  • The corresponding date of gestation can also show you how to calculate gestational age from the estimated due date.


Calculate Gestational Age

  • Most pregnancies last about 40 weeks from the first day of the last menstrual period.
  • The gestational age is based on the date of the last period, not the date of conception.
  • Because of this, a person is usually considered at least 4 weeks pregnant by the time they actually miss a period and have a positive pregnancy test.
  • To calculate your estimated due date and gestational age, enter the current date and the date your last menstrual period started.


Estimated Due Date

  • The estimated due date (EDD or EDC) is the date that the spontaneous onset of labor is expected to occur.
  • The due date may be estimated by adding 280 days ( 9 months and 7 days) to the first day of the last menstrual period (LMP).
  • This is the method used by "pregnancy wheels".
  • The accuracy of the EDD derived by this method depends on accurate recall by the mother, assumes regular 28-day cycles, and that ovulation and conception occur on day 14 of the cycle. 
  • Use of the LMP to establish the due date may overestimate the duration of the pregnancy and can be subject to an error of more than 2 weeks.
  • In cases where the date of conception is known precisely, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception.
  • Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the first day of the last menstrual period). 
  • The accuracy of the ultrasound estimate of the gestational age varies according to the gestational age. 
  • "Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age.
  • The Society of Obstetricians and Gynaecologists of Canada recommends the earliest ultrasound with a crown-rump length equivalent to at least 7 weeks (or 10 mm) should be used to determine the gestational age.


The American College of Obstetricians and Gynecologists recommends that ultrasound-established dates should take preference over menstrual dates when the discrepancy between ultrasound dating and LMP is

  • Greater than 5 days before 9 0/7 weeks of gestation by LMP
  • Greater than 7 days from 9 0/7 weeks to 15 6/7 weeks by LMP
  • Greater than 10 days from 16 0/7 weeks to 21 6/7 weeks by LMP
  • Greater than 14 days from 22 0/7 weeks to 27 6/7 weeks by LMP


Date of Conception

  • Because the human egg is capable of fertilization for only 12 to 24 hours after ovulation the date of ovulation may be taken as being the date of conception. 
  • However, ultrasound determination of the date of ovulation has the same imprecision as does the ultrasound estimate of the gestational age.
  • Therefore, a precise date of conception cannot usually be determined as with in vitro fertilization.
  • In addition, although a woman is most likely to become pregnant if she has sex on the day of ovulation conception may also occur from live sperm still in her reproductive tract on the day of ovulation if she had sex for up to five days before ovulation.


Observed Gestational Age at Delivery

The table below shows the average age of infants delivered in the United States according to the number of fetuses being carried (plurality).






Quintuplets and

higher-order multiples

Average Gestational Age (weeks)







Other Methods For Estimating the Gestational Age


Clinical Examination

  • A pelvic examination supported by good menstrual records in the first trimester has been reported to be a reliable method for dating of pregnancy.


Doppler Ultrasonography

  • The fetal heart can be heard using Doppler ultrasound by 10 to 12 weeks in most patients. 
  • The gestational age should, therefore, be at least 10 to 12 weeks if fetal heart tone is heard.


Human Chorionic Gonadotropin Pregnancy Test

  • Human chorionic gonadotropin first becomes detectable in the mother's blood and urine between 6 and 14 days after fertilization (3 to 4 weeks gestational age). 
  • The gestational age would, therefore, be at least 3 to 4 weeks at the time of a reliable hCG pregnancy test.


  • When a twin pregnancy is a result of in vitro fertilization determination of gestational age should be made from the date of embryo transfer. 
  • Otherwise, to avoid missing a situation of early intrauterine growth restriction in one twin, most experts agree that the clinician may consider dating pregnancy using the larger fetus.


Gestational Age Trimester of Pregnancy

  1. First Trimester of Pregnancy - Less than 14 weeks
  2. Second Trimester of Pregnancy - 14 weeks through 27 weeks 6 days
  3. Third Trimester of Pregnancy - 28 weeks through delivery


First Trimester of Pregnancy

  • During the first two weeks after the last menstrual period, egg follicles mature in the ovaries under the stimulus of follicle-stimulating hormone (FSH) a hormone secreted by the pituitary gland in the brain. 
  • High levels of the hormone estradiol, produced by the developing egg follicle,  cause the secretion of luteinizing hormone (LH), yet another hormone from the pituitary gland. 
  • LH causes the release of the egg from its follicle (ovulation).
  • For women with 28-day cycles, ovulation usually occurs on days 13 to 15.


Second Trimester of Pregnancy

  • The fetus's toenails are appearing. Gender may sometimes be seen.
  • The average fetus at 14 weeks is  5.8 inches (14.7 cm)  long (crown t to heel) and weighs 3.3 ounces (93 grams).


Third Trimester of Pregnancy

  • The average 41-week fetus is  20.5  inches (52 cm)  long and weighs  8.3  pounds  (3787 grams).



Overall, the gestational age calculator is a useful tool for healthcare providers to monitor and manage pregnancy and ensure the best possible outcomes for both mother and baby. Check More Medical Health Related Calculator on Drlogy Calculator to get exact health solution.



  • Gestational age by Wikipedia [1].
  • Gestational age by MedlinePlus [2].


favorite_border 2698 Likes

Gestational Age Calculator FAQ

What is normal gestational age?

  • The pregnancy is considered full-term at 37 weeks.
  • However, the fetus's lungs are usually fully developed at 34 weeks, allowing for a high chance of survival outside of the womb beyond that point.
  • Concluding, a baby born after 34 weeks of pregnancy may enjoy the same health as their peers born on term which is consoidered as normal gestational age.

How to calculate gestational age?

You can calculate the gestational age of a baby:

  • Using the last menstrual period date;
  • Assessing it with the ultrasound machine and fetal measurements; or
  • Applying the fertilization date, if we know it

How do we measure the time in gestational age?

  • Gestational age time is usually measured in weeks.
  • The entire pregnancy lasts around 40 weeks or 280 days.

The Power To Health

Copyright © 2023 Drlogy. All rights reserved.