What is an embryo?
An embryo is the early stage of a developing pregnancy, from about the second week after fertilization until the end of the eighth week after fertilization (about 10 weeks of pregnancy counted from the last period). In everyday use, including in IVF, "embryo" often means the fertilized egg from its first cell divisions onward. After this stage it's called a fetus.
From zygote to fetus
| Stage | Roughly when | What happens |
|---|---|---|
| Zygote | Day 0 to 3 after fertilization | Single cell, then early divisions |
| Blastocyst | Days 5 to 6 | Hollow ball of cells ready to implant |
| Implantation | Days 6 to 10 | Attaches to the uterine lining, starts making hCG |
| Embryo | Weeks 2 to 8 after fertilization | Heart, brain, spine and limbs begin to form |
| Fetus | From week 9 after fertilization | Organs grow and mature |
Why it matters
Pregnancy weeks are counted from the first day of your last period, about two weeks before conception. So "embryo at 6 weeks" usually means six weeks of pregnancy, roughly four weeks after fertilization. That mismatch confuses a lot of people.
In IVF, embryos grow in the lab for three to six days before transfer or freezing, and are described as day-3 or day-5 (blastocyst) embryos. The IVF due date calculator uses that age to date the pregnancy.
The embryo stage is the time most early losses happen, often because of chromosome differences no one could have prevented. If it happens to you, it is not your fault. See miscarriage.
In early pregnancy, get urgent care for heavy bleeding, severe pain, one-sided pain or fainting, which can be signs of miscarriage or ectopic pregnancy.
Frequently asked questions
When does an embryo become a fetus?
At the end of the eighth week after fertilization, about 10 weeks of pregnancy counted from the last period.
When does the embryo's heart start beating?
Cardiac activity can often be seen on a vaginal ultrasound at about 6 weeks of pregnancy.
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 a zygote?A zygote is the single cell formed when a sperm fertilizes an egg. How it becomes a blastocyst, when it implants, and how it fits into early pregnancy.
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: 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.
What is IVF (in vitro fertilization)?IVF (in vitro fertilization) joins eggs and sperm in a lab, then places an embryo in the uterus. The steps, who it helps, and how due dates work.IVF due date calculator: due date from your embryo transferIVF due date calculator: enter your embryo transfer date and embryo age (day 3, 5 or 6) for your estimated due date. Private, instant, nothing is stored.