Cycle Day 1

Definition - What does Cycle Day 1 mean?

Cycle day 1 is the first day of a menstrual cycle and is marked by the beginning of menstrual bleeding. A normal menstrual cycle is typically considered to be 28 days in length. For menstrual bleeding to count as the first day of the cycle, there should be a full flow of menstrual blood, not just mild spotting and it should occur before mid-afternoon. If a proper flow of menses is only seen after mid-afternoon, the next day is typically considered to be the first day of the cycle. Cycle day 1 is an important landmark as far as natural conception, fertility investigations and treatment of infertility are concerned.

FertilitySmarts explains Cycle Day 1

In the absence of fertilization of the egg, the corpus luteum slowly loses its function towards the end of the ovarian cycle, and the hormones produced by it, mainly progesterone and estrogen, decline to the point that the uterus is no longer able to hold the thick vascular inner layer that grew to support implantation of an embryo. The tissue breaks down and is shed as a menstrual period and a new menstrual cycle starts over again.

In a 28-day cycle, the release of the egg from the ovary usually occurs around 14 days after cycle day 1. There is a period of around 6 days in every menstrual cycle when the chances of a woman getting pregnant are considerably higher than the other days of the cycle, which is known as the fertile window. In a 28-day cycle, a fertile window typical falls during days 11 to day 14, so counting days from cycle day 1 and timing intercourse is extremely important to increase the chances of natural conception.

The measurement of hormones levels in blood for the evaluation of infertility is usually done in relation to a menstrual cycle so that the results can be better explained when the values correlate with the stage of the egg and the corpus luteum. FSH, LH, and estrogen levels are usually measured during day 2 to day 4 of the cycle and blood progesterone level is measured close to the 21st day of the cycle.

When a woman conceives, cycle day 1 is used to calculate the age of the fetus until a more accurate estimate is made with the help of ultrasound scanning.

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