Embryo creation lies at the heart of IVF (in vitro fertilization): an embryo is created under laboratory conditions from the intended parent’s sperm and egg, donor sperm, donor egg, or any combination thereof. Once created, embryos may be frozen for future use or transferred to the intended mother’s womb (or the womb of a surrogate). Here we’ll explain how embryo creation and transfer work, what percentage of fertilized eggs make it to birth, and other questions intended parents have about these important aspects of assisted reproduction.
What is an embryo?
When an egg is fertilized by sperm it creates a zygote, which combines the DNA of the two biological parents. The zygote grows by dividing into embryonic cells.
Once the zygote has divided into twelve or sixteen cells, it is called a morula. After about four days the morula forms a fluid pocket and is now called a blastocyst. The embryo is ready to attach to the uterine lining (the endometrium) in one to five days after becoming a blastocyst.
Embryo grading: Determining the success rate of a pregnancy
Embryo grading is an evaluation tool used during the 5-day blastocyst implantation timeline. IVF success rates depend in part on the quality and health of the embryo, so this is an important part of the process. Your doctor or embryologist will decide which embryos should be transferred, how many embryos to transfer, and the best time for the transfer to occur.
Embryo grading examines the appearance of the embryo, so the embryologist’s judgment is extremely important. Grading typically occurs on day 5 and 6 of embryo development at the blastocyst stage and examines the expansion of the blastocyst, and the quality of the future baby cells (inner cell mass) and future placenta cells (trophectoderm)
Embryo Grading Day 5
By day five the embryo is a blastocyst, and its cells have begun to grow beyond the zona pellucida or the “shell” of the egg. Blastocyst embryo grading considers three factors:
- The degree of expansion within the blastocyst cavity (expressed as a number between one and six, with six indicating the most expansion).
- The appearance of the inner cell mass (the part that become the baby), which receives a grade of A, B, or C.
- The appearance of the trophectoderm (the part of the blastocyst which becomes the placenta), which receives a grade of A, B, or C.
Embryo grading and success rates
Embryo grading is important, as it helps select the strongest and healthiest embryos for transfer. It is not, however, the only consideration for pregnancy success rates. The age of the mother or egg donor, the quality of the egg and sperm, and previous pregnancy histories all contribute to the embryo transfer outcome. Uterine abnormalities, fibroid tumors, and ovarian dysfunction also influence success rates.
How many embryos make it to day 5?
On average, between forty to fifty percent of fertilized eggs do not make it to day five. This is why more than one egg must be fertilized during IVF to ensure that at least one viable blastocyst embryo is available for transfer.
Embryo testing for gender selection and chromosomal abnormalities
Chromosomal abnormalities can interfere with pregnancy and result in children born with serious health problems. Chromosomal screening before embryo transfer (Preimplantation Genetic Screening, now called PGT-A) helps detect and avoid such problems and can be performed on a 5 and 6-day old blastocyst embryos. An embryologist will biopsy the embryo and safely remove 5 or 6 cells from the future placenta compartment (trophectoderm) that are then sent to a testing laboratory to screen for chromosomal abnormalities. At this stage, it is possible to reveal the embryo’s gender, allowing for sex selection, if the intended parents are hoping for a particular gender of their child.
Embryo testing (PGT-A) is an important part of the process and significantly increases the chances of successful IVF treatment, regardless of if the embryo is then implanted to an untended mother’s uterus, or that of a surrogate. Women who have passed the peak of their fertility have fewer eggs and these often give rise to chromosomal abnormalities. Embryos that develop with chromosomal errors almost always result in early miscarriage since few defects involving chromosomes are compatible with life. PGT-A improves the chance that any embryos used in IVF have a normal chromosome count and that there are no errors in the combination of chromosomes. The screening process is almost 100% accurate for identifying healthy, normal embryos. It is not intended to be a guarantee against birth defects since most defects that result in live birth are caused by other factors during the pregnancy. Instead, the purpose of PGT-A is to ensure that an embryo is viable and likely to thrive upon implantation.
It’s important to note that when donor eggs are used to create the embryo, embryo testing is usually not recommended since the eggs are donated by healthy young women and the chances of chromosomal abnormalities in the resulting embryos are low. Intended parents may still use PGT-A if desired especially if sex selection is planned.
How do frozen embryo transfers work?
Just before embryo transfer, the intended mother or surrogate is given oral medication to relax the pelvic muscles. A soft catheter is guided into the cervix by ultrasound. The embryo(s) are transferred to the uterus through the catheter. In most cases, the procedure is painless and does not require anesthesia. Some women experience mild cramping. After the embryo transfer, the woman must remain in bed for fifteen to twenty minutes and will need to arrange a ride home.
The risks during embryo transfer are minimal. Embryos can be lost during transfer or implanted in the wrong location, such as the fallopian tubes. The largest risk is the possibility of multiple pregnancies if two or more embryos are transferred, which can increase the risk of miscarriage and preterm delivery. To reduce this risk, Santa Monica Fertility typically transfers one embryo at a time.
Multiple embryo transfers can be of benefit to some women, including women who have experienced failed IVF cycles in the past, or reproductively older women who want to use their own eggs. Your doctor will be able to help you choose between a single embryo transfer and multiple transfers.
What happens after the Embryo Transfer: A Day to Day Guide
Body changes after an embryo transfer are subtle and unlikely to be noticed by the intended mother or surrogate at this early stage. Here’s a breakdown of what happens in the days after an embryo transfer:
- Day 1: The blastocyst begins to hatch from its shell
- Day 2: The blastocyst completes hatching and begins attaching itself to the uterus.
- Day 3: The blastocyst attaches deeper to the uterine lining, and implantation begins.
- Day 4: Implantation continues.
- Day 5: Implantation is complete. The cells that will become the placenta and fetus begin to develop.
- Day 6: Human chorionic gonadotropin (hCG), a hormone that indicates pregnancy, begins to enter the bloodstream.
- Day 7 and Day 8: hCG continues to enter the bloodstream and can be detected by a blood test; fetal development continues. Your doctor will draw blood on day 7 and again on day 9-10 after the embryo transfer to check hCG levels.
What percentage of embryos make it to live birth?
Success rates for mothers who want to use their own eggs depend on multiple factors, with the mother’s age being the most important. Women aged 35 to 37 who choose to use their eggs have a 40 percent success rate, while women over the age of forty have a 20 percent chance of success. At Santa Monica Fertility we see a success rate of 65 percent live birth per embryo transfer for patients who use donor eggs from our Donor Egg Bank.
To ensure the highest possible success rate with donor eggs, Santa Monica Fertility has a strict donor screening process in place. Egg donors must meet the following criteria:
- Donors must be between the ages of 18 and 30.
- Genetic screening must rule out common inheritable disorders.
- Donors must be in excellent health.
- Donors must be tested for STDs and other infectious diseases.
- Donors must undergo a medical examination and reproductive health evaluation.
Learn more about our best-in-class fertility and third party reproduction services
The doctors and team members of Santa Monica Fertility offer the best-in-class fertility services backed by decades of experience and research. We encourage you to explore more about surrogacy , IVF and egg donation through our fertility blog and to contact us with any questions you may have. You can call our patient coordinators on 310-566-1470.
