Cycle Day (CD)

Definition - What does Cycle Day (CD) mean?

The term "cycle day" refers to the day in a woman’s menstrual period. It starts with day one, the first day of menstrual bleeding, and usually lasts around 28-32 days, although a normal menstrual cycle can be shorter or longer. Tracking of a menstrual cycle day-by-day 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). The following is a basic breakdown of the menstrual cycle, highlighting the brief window that conception is most likely.

  • Cycle Day 1 (CD1) –The blood and tissues that were lining the walls of the uterus are shed as blood. This is the first day of menstrual bleeding.
  • CD2-5 – Menstrual bleeding, usually lightening by day 5.
  • CD7 – Small follicles in the ovaries are forming, each containing an egg (or sometimes two).
  • CD8-10 – The follicles in the ovaries are growing and preparing to release an egg. Only one main follicle will develop to maturity, and the uterus lining will continue to thicken with blood and nutrients so that it will be prepared for a fertilized egg to implant itself.
  • CD11-13 – 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.
  • CD14 – Ovulation – the release of the egg from its follicle into the fallopian tube – occurs. This is the best day for intercourse in the hopes of becoming pregnant.
  • CD15-16 – The egg begins its journey through the fallopian tube (where fertilization happens) with the potential of meeting sperm for fertilization.
  • CD17 – 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.
  • CD28 – The last day of the cycle. If fertilization and implantation did not occur, the menstrual period will start as the uterine lining sheds and the reproductive system prepares for another cycle, another ovulation, and another possible implantation.


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