This gestational age calculator determines probable ovulation, fertility window and the gestation age with characteristic fetal development in the pregnancy, week by week. There is more information below the form on how the gestation calculator works and on the dating of pregnancy methods.

The first day of your last menstrual period: *
Menstrual cycle length: *

How does this gestational age calculator work?

This health tool allows the dating of pregnancy by providing useful information about it based on simple menstrual cycle data.

The first day of the last period and the menstrual cycle length are used in the gestational age calculator to determine the following dates, very important in any pregnancy calendar:

■ Probable ovulation date – the day around which ovulation occurred;

Fertility window – period of 4 days before and after probably ovulation when conception is most likely to have occurred;

■ Estimated gestational age in weeks and day – this is accompanied by a description of fetal development during the indicated week;

Estimated due date – possible delivery date based on probably conception date and the usual 40-week estimate of standard pregnancy duration.

Establishing the gestational age of a pregnancy

The calculation method used above accounts for first day of menstrual cycle if known and considered conception to have occurred in day 14. Human gestation is considered to take 266 days from conception (or 280 from the last menstrual period).

There is also a simple indirect rule, Naegele’s rule that advises to add 7 days to the first day of LMP and subtract 3 months to get the EDD.

In case there are cycle irregularities or recent intake of contraceptive pill, working out the date from LMP may not be possible.

Physical exams can provide certain evidence of pregnancy date because the uterus can be felt coming over the pubic bone starting with 12 weeks and then at the navel at 20 weeks. Such an exam cannot always be performed accurately, i.e. because of obesity, multiple pregnancy etc.

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 to be the most accurate method to determine the gestational age of the pregnancy and to estimate due date.

The first scan is a dating scan (ultrasound measurement of the embryo/fetus) that usually takes place between the 11 and 13th week of pregnancy. Early scans, between 6 and 10 weeks might be offered in exceptional circumstances.

This scan focuses on dating the pregnancy and on whether the fetus is generally developing well. This is also when twin, triplet etc. pregnancies are discovered. The 20-week scan will focus on fetal development in more detail.

The importance of accurate establishing of pregnancy date is seen in the difference it makes when nearing due date and can prevent an up to date pregnancy to be considered overdue and induced.

Example of a pregnancy dating

For instance, in a pregnancy within a cycle of 29 days with the last period starting on February 2nd, 2016 and the current date being May 31st, 2016 the result is:

■ The probable ovulation date is February 18th, 2016;

■ The fertility window is between February 14th and February 22nd, 2016;

■ The estimated gestational age 16 weeks and 6 days;

■ The estimated due date is November 8th, 2016.

Week 17: To this week your baby is still tiny, measuring about 5.2 inches and weighing almost 5 ounces. Muscular and skeletal development continues. The placenta is continuing to develop in order to offer support to the vital functions of the baby. The placenta is a complex structure with many blood vessels used for feeding the baby with nutriments, providing the necessary oxygen and removing the waste products.


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

2) 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.

3) 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.

4) 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.

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

11 Jun, 2016