Best Due Date Calculator

Determines the probable ovulation, conception fertile window and estimated due date.

The due date calculator uses the first day of the menstrual cycle and the average menstrual cycle length to provide three pieces of information, important for dating pregnancy:

■ Probably ovulation date;

■ Fertile window (likely when conception occurred 5 days before and 2 days after ovulation);

■ Estimated due date (approximately 266 days from conception).

About menstrual cycle & ovulation

The menstrual cycle occurs in fertile females every 28 to 33 in average, but there are instances of shorter or even longer cycles, between 26 and 36 days. Each cycle consists of three different phases: the follicular one, ovulation and the luteal one, all controlled by the endocrine system.

The follicular phase starts on day 1 of menstrual bleeding, which marks the beginning of a new cycle. During it, follicles begin developing, ready to be released from the ovary when mature enough, which marks the second phase: ovulation.

Ovulation occurs around half-way through the menstrual cycle, but this is not a given rule, in shorter cycles, for example, ovulation can occur earlier.

The fertile window is considered given egg and perm viability and ranges between 5 days before ovulation and 2 days after ovulation.

The events in the luteal phase depend on whether the egg has been fertilized or not – if fertilization occurs, the egg implants in 6 to 9 days in the uterus lining and continues to develop. If fertilization does not occur, the uterus prepares to shed its lining and the unfertilized egg in around 14 days after ovulation, causing the period bleed and for a new menstrual cycle to begin.

The estimated due date accounts for a birth at exactly 40 weeks and is estimated as 266 days from conception. However, it is important to note that 20% of women give birth in week 38, 20% in week 39, 30% give birth in week 40 and 20% in week 41.


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

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


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