What Does Cycle Day (CD) Mean?
The term "cycle day" refers to a specific point in a woman’s menstrual cycle. Cycle day starts with day one of a period, which is the first day of bright red bleeding, and usually lasts around 21-35 days.
Tracking of a menstrual cycle day-by-day along with monitoring physical characteristics like basal body temperature and cervical mucus can help to pinpoint the brief window that a woman is fertile and most likely to become pregnant.
FertilitySmarts Explains Cycle Day (CD)
A menstrual cycle is controlled by the fluctuating levels of several hormones (FSH, LH, estrogen, and progesterone) over three phases. The three phases mark the development of follicles, which contain eggs (follicular phase), the release of a mature egg (ovulatory phase) and the luteal phase where progesterone either helps to sustain a fertilized egg or its absence triggers the shedding of the lining and the start of the next cycle.
There is a wide variation of what is considered to be a normal menstrual cycle, but the following is an approximate breakdown of a typical menstrual cycle might look like by cycle day.
Cycle Day 1 (CD1)
The blood and tissues that were lining the walls of the uterus in preparation for pregnancy that did not occur are shed as blood. This is the first day of menstrual bleeding or a period.
Menstrual bleeding occurs, usually lightening by day 5. Some may bleed as long as 8 days.
Small follicles in the ovaries are forming, each containing an egg. The uterine lining starts to become thick.
The follicles in the ovaries are growing and preparing to release an egg. Only one dominant follicle will typically develop to maturity, and the uterine lining will continue to thicken with blood and nutrients so that it will be prepared for a fertilized egg to implant itself.
Ovulation will be happening soon. The egg can survive for 24 hours and the sperm for 72, so to get the sperm and egg in the ideal place for fertilization, intercourse is advised between now and day 14.
Ovulation — the release of the egg from its follicle into the fallopian tube—occurs somewhere around day 14 for people with a 28-day cycle. Ovulation does not occur consistently on day 14 for different cycle lengths.
The egg begins its journey through the fallopian tube (where fertilization happens) with the potential of meeting sperm for fertilization.
The lining of the uterus has reached peak thickness. If the egg was fertilized, it will soon embed itself in the lining and pregnancy will begin. If the egg wasn’t fertilized by sperm within the first 24 hours after ovulation, it will be passed from the body.
The last day of the cycle. If fertilization and implantation did not occur, hormonal changes will trigger the start of a period as the uterine lining sheds and the reproductive system prepares for another round of ovulation and another possible pregnancy.