This due date calculator determines the probable ovulation day in your cycle, the dates of conception and the estimated due date. Read below more on how this tool works and about the menstrual cycle important dates.

The first day of your last menstrual period:*

Menstrual cycle length:*

How does this due date calculator work?

This useful tool can determine the probable ovulation date, the fertility window for conception and an estimated due date in case of a pregnancy occurring all using just simple data from the menstrual cycle.

There are only two key elements that are considered for this calculation are the length of the menstrual cycle, that can very from 28 to 33 days and the first day of the last period.

Then the due date calculator uses the basic information on female reproductive system to set the probable date of the ovulation, the window of fertility and the estimated due date. You can make as many determinations as you like just by pressing calculate again.

Example due date calculation and result:

For a menstrual cycle of 28 days with the first day of the last period being September 9th 2014 the result is:

■ The probable ovulation date is September 24th, 2014.

■ The possible dates of conception are between September 20th and September 28th, 2014.

■ The estimated due date is June 16th, 2015.

The probable ovulation date is considered day 14 of the menstrual cycle while 4 days before and after this date are considered in the fertility window for the dates of conception. The due date is set at 40 weeks.

The menstrual cycle

The menstrual cycle defines the changes in the female body when is being prepared for reproduction. For that is essential for the ovaries to produce eggs and for the uterus to be ready to host an embryo. A woman's first menstruation occurs around the age 13 and is called menarche. The end of a woman's reproductive period is called menopause and occurs between the ages of 45 and 55.

The menstrual cycle occurs in fertile females every 28 days, in average. The length may vary between 26 and even 34 days. Each cycle consists of three different phases, controlled by the endocrine system: follicular phase, ovulation and luteal phase. 


By convention, each menstrual cycle is considered to begin from the first day of menstrual bleeding. In the first phase, follicles in the ovary begin developing and after several days, approximately in the middle of the cycle, one of them is mature enough to be released from the ovary. Event called ovulation.

So the probable ovulation date that the form above shows is the moment, half way through the menstrual cycle, in which the female body releases a mature egg from the ovary, egg that can be fertilized if it meets a male sexual cell. If the fertilization occurs, the egg will implant in the uterus lining and continue developing in the future new born.

If fertilization does not happen, the uterus will shed its lining and egg in about two weeks after the ovulation, causing the bleeding period called menstruation.

The possible conception dates represents the most fertile period of time in which if a sexual intercourse occurs the egg is most likely to be fertilized and generate a pregnancy. This period of time covers from about 5 days before ovulation until 1-2 days after ovulation. 

Estimated due date

Moreover the estimated due date is the date in which the birth will take place in the event of a normal pregnancy that began in one of the conception dates. This due date is an estimate of approximately 266 days from the conception.

It should also be taken into consideration that not all pregnancies are terminated with birth at exactly 40 weeks. Statistics say that 20% of women give birth in week 38, 20% in week 39, 30% give birth in week 40 and 20% in week 41.


1) Beckmann CRB. ed. (2010) Obstetrics and Gynecology. Lippincott Williams & Wilkins. pp. 306–307

2) Abman SH. (2011) Fetal and neonatal physiology (4th ed.) Philadelphia: Elsevier/Saunders.

28 Dec, 2014