Close button
Cofertility and &Mother are proud to introduce Level the Playing Field, an initiative to help promote fertility freedom for women in sports
Cofertility Logo
Find your match
About
About UsWhy CoBaby GuaranteeLGBTQ+Why Egg Sharing
LearnFAQsPricing
LoginFind your match
Family by Co favicon
Learn
/
IVF Embryo Development by Stages
IVF

IVF Embryo Development by Stages

by
Dr. Meera Shah
May 22, 2024
Last updated:
October 31, 2024
Graphic of the embryo development stages from day 1 to day 5. Going from fertilization, zygote, 2 cell stage, 4 cell stage, 8 cell stage, Morula 16 cell stage, Morula 32 cell stage, and blastocyst.

Starting with a fertilized egg, embryo development is a complex biological phenomenon that has fascinated scientists for centuries. With advances in assisted reproductive technology (ART), particularly in vitro fertilization (IVF), our understanding of embryo development has expanded dramatically — contributing to the birth of 91,771 ART-conceived babies in 2022! 

This article walks through the step-by-step processes involved in unassisted and assisted embryo development, highlighting underlying biological events, how IVF works, and the factors influencing fertilization success. Let’s jump in.

It starts with the egg 

Ovulation is the starting point of embryo development. It is the part of the menstrual cycle when one ovary releases a mature egg. Once it is released, the egg moves down into one of the fallopian tubes, where it can survive for about 24 hours. This can result in pregnancy if the egg gets fertilized by sperm during that time – if it doesn’t get fertilized, the egg will break down and eventually will shed along with the lining of the uterus when you have your period.  This typically occurs around the 14th day in a 28-day cycle, though not for everyone. The day of ovulation varies based on cycle length, anything ranging 21 - 35 days is considered normal. Hormones such as luteinizing hormone (LH) and follicle-stimulating hormone (FSH) play critical roles in inducing ovulation. These hormones are always in your body, but your FSH levels fluctuate throughout the menstrual cycle, typically peaking just before LH levels peak to trigger ovulation.

During an egg freezing / IVF cycle, understanding the timing of your menstrual cycle is important as it influences the scheduling of your treatment. Ovulation is carefully controlled through the administration of hormonal drugs that stimulate the ovaries to produce multiple eggs and bring them to peak maturity. The development of these eggs is monitored through regular ultrasounds and blood tests, which are crucial to ensure the specific medication protocol you are given is personalized to you. Once the eggs are mature, they are retrieved surgically under anesthesia.

Then, the eggs are fertilized 

Fertilization marks a pivotal stage of embryo development. During unassisted embryo fertilization, it occurs in the fallopian tubes and begins when a sperm cell breaks through the outer layer of the egg, the zona pellucida. This penetration triggers reactions that prevent other sperm from entering the egg. Then, the nuclei of the sperm merge with the nuclei of the egg, combining their genetic material to form a zygote, the earliest stage of the embryo.

During IVF, fertilization takes place in a lab setting, using the retrieved eggs and sperm. An embryologist combines each egg with sperm, under a controlled, microscopic watch to monitor daily development. Ideally, the same response occurs as during an unassisted fertilization process. To enhance success rates, especially in cases of male factor infertility and low sperm counts, a single sperm is injected directly into the egg using a technique called intracytoplasmic sperm injection (ICSI). 

The early embryo develops through cleavage 

Once the fertilized egg has created a zygote, it then begins a series of rapid divisions in a process known as cleavage. This stage of embryo development involves the embryo splitting without growing in size, essentially forming a compact ball of cells. Cleavage is vital as it transforms the single-cell zygote into a multicellular entity, setting the foundation for further differentiation and growth. Both in unassisted conception and IVF, studies have shown the quality and speed of these divisions can be an early indicator of the embryo's health and viability.

The blastocyst forms 

About five days after fertilization, the developing embryo hopefully(!) reaches the blastocyst stage. This is when the embryo is made up of two parts: a cluster of cells inside that will develop into the baby (called the inner cell mass) and a shell on the outside that will help form the placenta (called the trophoblast). 

During unassisted conception, the blastocyst travels from the fallopian tube to the uterus throughout this five day period. During IVF, however, the blastocyst first develops within the lab and then is frozen. At this point, many families opt for genetic testing of the embryo. 

Preimplantation genetic testing for aneuploidy (PGT-A) allows families to know whether embryos possess the right number of chromosomes. A “euploid” embryo is genetically healthy enough to transfer. Embryos with chromosomal additions or deletions are considered aneuploid. Mosaics are somewhere in between and we’re still learning more about their potential. 

PGT-M (preimplantation genetic testing for monogenic disorders), on the other hand, takes place if there is a specific genetic diagnosis for which both reproductive partners are carriers. This technology allows embryologists to identify embryos possessing those specific genetic abnormalities. That way, parents can choose to avoid transferring one of these embryos if it would put the future child at risk. 

A healthy embryo is transferred and grows in the uterus

Your PGT results (should you choose to pursue that testing), combined with the grades given to each of your blastocyst embryos, will both factor into your and your doctor’s decision on which embryo would be best suited for transfer. 

During IVF, an embryo transfer procedure is done with great care under ultrasound guidance to optimize the placement of the embryo(s) within the uterine cavity, enhancing the chances of successful implantation. The timing of this transfer is crucial and is carefully synchronized with the recipient's menstrual cycle to ensure the uterine lining is optimally receptive. 

Implantation and fetal development

Ideally, the hope is that the transferred blastocyst will attach itself to the uterine wall— a process known as implantation. During implantation, the outer layer of the blastocyst, known as the trophoblast, connects itself to the uterine lining. As we mentioned, timing is everything — implantation success is influenced by the hormonal balance of the body and the receptivity of the uterine lining, which needs to be thick enough to “accept” the embryo. This will eventually grow into placenta, and will be an essential connection for the transfer of nutrients and oxygen to the embryo as it develops into a fetus and, ultimately, a baby. 

The heart is the first organ to develop within an embryo — by the sixth week of pregnancy, the embryo may have developed an actual heartbeat, which can be measured on ultrasound.

Factors influencing embryo development success

Several factors affect the success of both unassisted and assisted embryo development:

  1. Age and quality of eggs: Older women tend to have fewer eggs, and the quality of eggs decreases with age, affecting fertilization rates and embryo development. Utilizing donor eggs can significantly improve outcomes in IVF treatments.
  2. Sperm quality: Parameters such as sperm count, motility, and morphology are crucial in both unassisted and assisted fertilization processes.
  3. Clinic and lab quality: Not all fertility clinics are created equal, and the same goes for labs. If you have not yet begun the IVF process, ask your clinic about their blastocyst development success rates (it’s key to use this language, as opposed to fertilization rate, for example) and feel free to call other local clinics and do the same. 
  4. Embryonic development: The ability of an embryo to reach the blastocyst stage is a key indicator of its viability and potential for successful pregnancy.

A recent study examined a few reasons why embryo growth may stall — in summary, this is largely due to chromosomal fragility and abnormalities. If possible, we recommend undergoing genetic carrier screening before pursuing IVF so as to make the experience as successful as possible and ideally provide a healthy pregnancy. 

Summing it up

In conclusion, the path from egg to blastocyst formation is pretty incredible and requires the completion of several successful steps even before a transfer attempt. As technological capabilities expand within the field of embryology, so too does the potential to assist those facing challenges in unassisted conception. 

If you’re interested in working with an egg donor to help create embryos and start or grow your family, click here to browse our platform with tons of egg donors eager to help. 

FAQs about embryo development

What percentage of fertilized embryos make it to day 5?

About one-half of fertilized embryos continue developing to become a day 5 blastocyst. When an embryo is created using donor eggs, this stat jumps to 60%. 

How long does it take for an embryo to develop after IVF?

In IVF, embryos are typically cultured in the laboratory for 5-6 days to reach the blastocyst stage, which is considered optimal for transfer. However, occasionally embryos may be transferred earlier, at the cleavage stage (day 3). 

Where does the embryo develop?

In unassisted conception, the embryo develops in the uterus. After fertilization, the embryo travels down the fallopian tube and reaches the uterus, where it undergoes several stages of development starting from a zygote to a blastocyst. Successful implantation into the uterine wall is essential for the continued growth and development of the embryo.

While the implantation process remains the same for an embryo’s development during IVF, the actual fertilization process for assisted conception takes place within a lab, where an embryologist combines an egg with sperm under close watch. Only once the embryo has spent 5 days developing into a blastocyst is it transferred into the recipient’s uterus, or may be frozen for a future embryo transfer cycle. 

What is the first organ to develop in the embryo?

The first organ to develop in the embryo is the heart. The heart begins as a simple tube-like structure and starts beating and pumping blood by around the 22nd day after conception, which is essential for the circulation of nutrients and oxygen to the developing embryo.

When does an embryo develop a heartbeat?

An embryo typically develops a heartbeat by the end of the fifth week of pregnancy, which is about three weeks after fertilization. This milestone can often be seen during an early ultrasound scan, and is a crucial indicator of the embryo's viability.

What is preimplantation genetic testing (PGT)?

PGT is a process that screens embryos created through IVF for specific genetic conditions or chromosomal abnormalities. This can help increase the chances of a healthy pregnancy and reduce the risk of miscarriage. Read more about PGT testing here.

Does using frozen eggs affect embryo development?

Research suggests that eggs frozen through vitrification have similar success rates to fresh eggs in IVF. However, the success rates of utilizing frozen eggs still depends on the age of the egg when it was frozen. The younger the egg(s), the greater its potential for successful fertilization and development into a healthy embryo.

Dr. Meera Shah

Meera Shah, MD, FACOG, is a double board-certified OBGYN and reproductive endocrinology and fertility specialist at NOVA IVF in Mountain View, California. She is a Founding Medical Advisor at Cofertility. Dr. Shah has authored numerous research articles on topics ranging from fertility preservation, pregnancy loss, reproductive genetics, and ethnic differences in IVF outcomes. Her medical practice incorporates the highest level of evidence-based medicine and the most cutting edge technologies to optimize outcomes for her patients. Dr. Shah applies this approach to her work with Cofertility, ensuring that Cofertility remains up-to-date on latest medical advancements and research in third-party reproduction and reproductive endocrinology in general. When Dr. Shah isn’t busy working with her patients at NOVA IVF, she enjoys playing pretty much any sport, learning new piano pieces on YouTube, and spending quality time with her husband and three boys. You can find her on Instagram providing fertility-related advice and education at @dr_meerashah.

Read more from
Dr. Meera Shah
Tags
IVF
embryo donation
Share
Twitter icon
Facebook icon
LinkedIn icon

Keep learning

View all
Photo of a written schedule of a day in the life of an IVF patient
IVF

The Emotional Labor of Managing Fertility Medications: Why Organization Matters

The success of a fertility treatment cycle is dependent on the medication - the types of medication, dosages, frequency, etc. So it is no wonder that managing fertility medications can create a psychological strain causing stress, anxiety, frustration, and emotional exhaustion. Learn more about how you can help keep yourself organized through the process.

Read more
Instagram icon
an abstract painting with orange, yellow, and blue hues
IVF

POI, POF, POR, DOR, FOR, TOR, Oh My! A Guide to Ovarian Reserve Acronyms

Each of these acronyms represents a distinct condition or factor that can influence fertility, and understanding them is vital to making informed decisions about your reproductive health.These acronyms pertain to various ovarian conditions that can contribute to infertility, each with their unique challenges and treatment considerations. Though these conditions can introduce obstacles on the journey to parenthood, modern advancements in reproductive medicine provide promising solutions. In particular, egg donation stands as a beacon of hope for many individuals and couples navigating these challenges.

Read more
Instagram icon
Woman in socks helping baby walk on carpet.
IVF

What You Need to Know About Getting Pregnant In Your 40s

If you are over 40 and trying to grow your family, you may be wondering what path gives you the best chance for success. We're diving into the data around your possible paths to pregnancy.

Read more
Instagram icon
Photo of a written schedule of a day in the life of an IVF patient
IVF

The Emotional Labor of Managing Fertility Medications: Why Organization Matters

The success of a fertility treatment cycle is dependent on the medication - the types of medication, dosages, frequency, etc. So it is no wonder that managing fertility medications can create a psychological strain causing stress, anxiety, frustration, and emotional exhaustion. Learn more about how you can help keep yourself organized through the process.

Read more
Instagram icon
an abstract painting with orange, yellow, and blue hues
IVF

POI, POF, POR, DOR, FOR, TOR, Oh My! A Guide to Ovarian Reserve Acronyms

Each of these acronyms represents a distinct condition or factor that can influence fertility, and understanding them is vital to making informed decisions about your reproductive health.These acronyms pertain to various ovarian conditions that can contribute to infertility, each with their unique challenges and treatment considerations. Though these conditions can introduce obstacles on the journey to parenthood, modern advancements in reproductive medicine provide promising solutions. In particular, egg donation stands as a beacon of hope for many individuals and couples navigating these challenges.

Read more
Instagram icon
Woman in socks helping baby walk on carpet.
IVF

What You Need to Know About Getting Pregnant In Your 40s

If you are over 40 and trying to grow your family, you may be wondering what path gives you the best chance for success. We're diving into the data around your possible paths to pregnancy.

Read more
Instagram icon
Left arrow icon
Right arrow icon
View all articles
Instagram icon
X icon
LinkedIn icon
About Cofertility
  • Contact
  • Careers
  • For Clinics
  • Press
  • LGBTQIA+ Support
Our ecosystem
  • Freeze by Co
  • Family by Co
Label
While our egg freezing programs are for people with ovaries, we respect the reproductive identity, rights, and choices for all.
Cofertility logo
This site is for informational purposes only and not a substitute for medical advice, diagnosis, or treatment. If you're looking for medical advice, we suggest talking with a qualified physician.
© 2025 Cofertility. All Rights Reserved. Terms of Use. Privacy Policy.
Let's keep in touch.
Thanks for submitting your email!
We’re excited to keep in touch.
Something went wrong while submitting the form. Please try again.
By submitting your email you agree to our Terms of Use and agree that we can contact you as described in our Privacy Policy.
Member of the ASRM (American Society for Reproductive Medicine) logoLGBTQ+ Friendly BusinessResolve Proud Partner logoProud to be Women-Owned

By using Cofertility's sites, you agree to our use of cookies to enhance your browser experience, personalize offers and content, and analyze our traffic. Learn more in our Privacy Policy.

Accept