Egg (ovum): definition and meaning
An ovum, or egg, is the female reproductive cell. A mature egg is released from an ovary at ovulation and can be fertilized by a sperm to form a zygote. It is one of the largest cells in the human body, just visible to the naked eye, and survives only about 12 to 24 hours after release.
From oocyte to ovum
Eggs are stored in the ovaries from before birth, as immature cells called oocytes, each inside its own follicle. Each cycle, hormones wake up a small group, one matures fully, and it is released at ovulation. Strictly speaking, scientists call it an ovum once it has finished maturing, but in everyday use "egg", "oocyte" and "ovum" mean the same thing.
Key numbers
| Approximate figure | |
|---|---|
| Eggs at birth | 1 to 2 million |
| Eggs at puberty | 300,000 to 400,000 |
| Eggs ovulated in a lifetime | About 400 to 500 |
| Lifespan after ovulation | 12 to 24 hours |
| Size | About 0.1 mm across |
What happens next
After release, the egg is swept into a fallopian tube. If it meets sperm there, it may be fertilized and become a zygote, which divides into an embryo. If not, it breaks down and is reabsorbed, and a period follows about two weeks later.
Eggs and age
Both the number and the quality of eggs fall with age, faster after about 35. That's the main reason fertility declines and why the chance of chromosomal differences rises. The remaining supply is called ovarian reserve.
Frequently asked questions
How long does an egg survive after ovulation?
About 12 to 24 hours. Sperm can live up to 5 days, which is why the fertile window starts before ovulation.
Do you lose an egg with every period?
You release about one egg per ovulatory cycle, but many more immature eggs are naturally lost each month whether or not you ovulate.
Can more than one egg be released?
Sometimes two are released in the same cycle, which can lead to fraternal twins.
Sources
- Cleveland Clinic: Ovulation
- Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. NEJM (1995)
This page is for information only and isn’t medical advice. If something worries you, please talk to a doctor.
Keep reading
Ovulation: definition and meaningOvulation is when an ovary releases an egg, usually 12 to 14 days before your next period. Signs, timing, the fertile window, and how it affects mood.
Ovaries: definition and meaningThe ovaries are two small glands that store eggs and make estrogen and progesterone. What they do across the cycle, through life, and why they matter.
Fertile window: definition and meaningThe fertile window is the six days each cycle when sex can lead to pregnancy. How it's worked out, when it falls, and why it isn't a contraceptive tool.
Ovulation phase: signs, timing and feelingsThe ovulation phase explained: when you ovulate, how long it lasts, the signs and symptoms to look for, and why mood, energy and libido often peak.Ovulation calculator: when do I ovulate?Free ovulation calculator: enter your last period and cycle length to see your likely ovulation day for the next 4 cycles. Private, instant, estimates only.