Gestational Age Calculator

Estimates the gestational age of the pregnancy and displays weekly fetal development information, along with the due date.

The first day of your last menstrual period:
Menstrual cycle length:
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.

Pregnancy dating – establishing gestational age of a fetus

This gestational age calculator offers information on the probable ovulation date and fertility window (when conception likely took place), the estimated gestational age in weeks and days with a description of current fetal development and the estimated due date based on a 40-week delivery.

Human gestation is estimated to take 266 days from conception date or 280 days from the last period.

Naegele’s indirect rule for EDD advises to add 7 days to the first day of LMP and subtract 3 months to get the EDD. Last menstrual period dating must be made with caution as cycle irregularities may make the prediction inaccurate.

Best dating of pregnancy is done through physical exams and ultrasound. During a pelvic physical examination, in pregnancies of over 12 weeks the uterus can be felt coming over the pubic bone and over 20 weeks will reach navel level.

Human Chorionic Gonadotropin (HCG) blood test can provide pregnancy age indication based on HCG levels occurring at 3 to 4 weeks.

Dating scans are considered the most accurate pregnancy dating method with the first scan usually occurring between weeks 11 and 13th of pregnancy in most health systems. The focus of this scan is to check whether the pregnancy is multiple, whether the fetus is developing well, to determine the gestational age through fetal measurements. The second scan, at 20 weeks, focuses on fetal development in more detail.

Accurate pregnancy dating is essential in being able to monitor the development of the pregnancy against guidelines but also with respect to due date and delivery, allowing clinicians to make informed decisions should inducing be required.


Sadler TW, Langman J. (2012) Medical embryology. 12th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.

Mul T, Mongelli M, Gardosi J. (1996) A comparative analysis of second-trimester ultrasound dating formulae in pregnancies conceived with artificial reproductive techniques. Ultrasound Obstet Gynecol; 8(6):397-402.

Hunter LA. (2009) A Simple Solution to Dating Discrepancies: The Rule of Eights. Issues in Pregnancy Dating: Revisiting the Evidence. Journal of Midwifery & Women's Health 54 (3): 184–190.

Westerway SC, Davison A, Cowell S. (2000) Ultrasonic fetal measurements: new Australian standards for the new millennium. Aust N Z J Obstet Gynaecol; 40(3):297-302.

Altman DG, Chitty LS. (1997) New charts for ultrasound dating of pregnancy. Ultrasound Obstet Gynecol; 10(3):174-91.

Specialty: Obstetrics Gynecology

System: Reproductive

Article By: Denise Nedea

Published On: April 14, 2020

Last Checked: April 14, 2020

Next Review: April 14, 2025