What is a zygote?
A zygote is the single cell formed when a sperm fertilizes an egg, usually in a fallopian tube within a day of ovulation. It contains genetic material from both parents. Within about a day it starts dividing as it travels toward the uterus, becoming a ball of cells, then a blastocyst that implants around 6 to 10 days after ovulation.
From zygote to embryo
| Days after fertilization | Stage |
|---|---|
| 0 | zygote: one cell, fertilization in the fallopian tube |
| 1 to 3 | cell divisions: 2, 4, 8 cells |
| 3 to 4 | morula: a solid ball of cells entering the uterus |
| 5 to 6 | blastocyst: a hollow ball with an inner cell mass |
| 6 to 10 | implantation into the uterine lining |
| From about 2 weeks | called an embryo until the end of week 8 of development |
Why the zygote stage matters
Many fertilized eggs never implant, often because of chromosome differences. This is a normal part of human reproduction and usually goes unnoticed, as the next period arrives on time. Some losses after implantation show only as a brief positive test, called a chemical pregnancy.
Where this fits in your cycle
Fertilization can only happen in a short window: the ovum survives about 12 to 24 hours after ovulation, while sperm can wait up to five days. That is why the fertile window is about six days long. During the two-week wait the zygote is quietly dividing, and nothing you feel yet can confirm it.
Pregnancy tests detect hCG, which only starts once implantation begins. That is why testing too early can give a negative even if fertilization happened.
Frequently asked questions
Is a zygote the same as an embryo?
No. The zygote is the first single cell. It becomes an embryo after implantation, around two weeks after fertilization.
Can you feel when a zygote forms?
No. Fertilization and early cell division cause no symptoms you can feel.
Sources
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
What is an embryo?An embryo is the developing pregnancy from about two weeks after fertilization to the end of week 10. What happens at this stage and how it's counted.
Conception: definition and meaningConception is when sperm fertilizes an egg, the start of pregnancy. How it happens step by step, when it can happen in your cycle, and how long it takes.
Implantation: definition and meaningImplantation is when a fertilized egg attaches to the lining of the uterus, usually 6 to 10 days after ovulation. What happens, timing and signs.
Conception calculator: when did I conceive?Conception calculator: enter your due date to find your likely conception date (due date minus 266 days) and the window of days the sex could have happened.
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.