Knowledge Sharing
Embryo Size Guide: From Gestational Sac (5 weeks) to Fetus Development
Upon discovering the little life growing inside you, the baby’s health becomes a constant concern for expectant parents. During the early stages of pregnancy, when the embryo is relatively unstable, its size is a crucial indicator of developmental progress, making it one of the most concerning aspects for many.
What is a normal embryo size during pregnancy?
Does embryo size impact the baby’s health?
Could a smaller embryo lead to a miscarriage?
Let’s start by understanding the process of embryo development and then explore the normal size ranges for embryos during weeks 5 to 8 of pregnancy.
Embryo Size Q&A
How Does the Embryo Develop? Clarifying the Names for Embryos at Different Weeks
Embryo development progresses through several stages: the union of sperm and egg, the formation of a zygote, its development into an embryo, and its growth into a fetus.
When an egg is successfully fertilized, a zygote forms within approximately 16 to 18 hours. The zygote then undergoes rapid cell division,multiplying exponentially until approximately the 5th day, when it forms a blastocyst.
The blastocyst is the stage before embryo implantation. The cells within the blastocyst typically develop into an embryo around the 4th week after fertilization, entering the embryonic stage, and eventually becoming a fetus by the 9th week.
What is the difference between a blastocyst, gestational sac, and embryo?
- Blastocyst: The ball of cells forms about 5 days after the fertilized egg begins cell division. This stage occurs before the embryo implants in the uterus. The trophoblast cells on the outer layer of the blastocyst provide the nutrients necessary for embryo growth. If abnormalities occur during cell division, development may be delayed or halted, leading to a condition known as a "blighted ovum."
- Embryo: After the inner cells of the blastocyst continue to grow for several weeks, they form the embryo, which is surrounded by the sac-like structure and will further develop into a fetus.
- Gestational Sac: The sac-like structure that surrounds the embryo, providing a protective environment and serving as the "home" for nurturing the developing embryo.
Does the size of the embryo affect the baby's health?
While the size of the embryo reflects the actual developmental status, early-stage embryo growth is naturally more unstable and does not directly indicate the embryo’s health.
Some expectant mothers may experience situations where the embryo is too small to be detected via abdominal ultrasound and requires a transvaginal ultrasound instead. This is a normal occurrence.
If the embryo appears smaller than expected for its gestational age, it could be due to later-than-estimated conception or signs of poor development or shrinkage. However, if the doctor assesses that the size is within a safe range, the expectant mother should not worry too much. Simply continue with regular prenatal checkups to monitor the embryo's growth.
How Your Embryo Grows: Size Chart (Weeks 4 to 8)
After the fertilized egg develops for the first three weeks, it implants in the uterus during the fourth week and becomes an embryo.
As the pregnancy progresses, expectant mothers naturally wonder about their baby's growth in the womb and what size is considered normal.
During the embryonic stage (Weeks 4 to 8), the embryo undergoes weekly changes. You can refer to the 'Embryo Growth Chart' below to learn about the average size and development of the embryo during this period.
Embryo Growth Chart (Weeks 4 to 8)
| Week | Average Length(cm) | Embryo Growth Status |
| 4 | 0.2 | Shortly after the embryo implants in the uterus, its size is still very small and might not be visible on an ultrasound. |
| 5 | 0.4 | The embryo's heart begins to beat, and the small gestational sac can be seen through ultrasound. |
| 6 | 0.85 | Ultrasound can clearly detect the gestational sac. During this time, the brain, nervous system, and organs begin to develop. |
| 7 | 1.3 | The basic human form begins to emerge, as various organs develop rapidly. In some embryos, the limbs positions become identifiable. |
| 8 | 1.6 | The baby's fingers and toes can be slightly seen, and most of the organs have developed and taken shape. The embryo's form shows a head larger than the body. |
Insights About Embryos: What You Need to Know
Is the embryo considered life?
The embryo is a living entity. Once the fertilized egg reaches the fourth week and enters the embryonic stage, the embryo will start to show a heartbeat, marking the transition from the cellular stage.
Is the size of the gestational sac important?
The gestational sac shows before the embryo implants, and it may not even be detectable in early pregnancy tests. During this time, the sac is still very small, so there is no need to be overly concerned with its exact size.
What are the signs of an unhealthy embryo?
An unhealthy embryo may cause it to shrink, leading to a miscarriage after the cessation of the heartbeat. If the embryo does not have a heartbeat by the eighth week, or if there are symptoms like abnormal vaginal bleeding, lower back pain, or lower abdominal pain, it's important to visit a doctor for an ultrasound to check the embryo's development.
What does the embryo look like when it is expelled?
The chance of a natural miscarriage in the early stages of pregnancy is about 30%.
When an embryo naturally expels, it is often enclosed in blood clots and expelled along with the blood. It may be mistaken for a regular blood clot, and some people who are unaware of their pregnancy may mistake it for a period.
At what size does the embryo have a heartbeat?
The embryo typically starts to have a heartbeat around the fourth week.
By the eighth week, if no heartbeat is detected, it may indicate the embryo has stopped developing.
Lee Women's Hospital: Protecting the Health of Your Embryo
Whether you're an expectant mother encountering challenges during pregnancy and unsure of what to do, or a couple struggling with infertility and considering assisted reproductive methods, Lee Women's Hospital is here to provide expert care for all your reproductive health needs.
As the hospital with the highest number of IVF cycles in Taiwan, LWH is proud to have a team of highly experienced doctors and skilled embryologists. From IVF implantation to successful pregnancy, our comprehensive one-stop services ensure you receive the best care and complete peace of mind.
When it comes to pregnancy, trust LWH to provide expert guidance and personalized care every step of the way.
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