It’s been over three decades since the first birth from a frozen egg was achieved. As birth rates from techniques like egg vitrification and IVF increase, the paramount question becomes: Do these techniques have any impact on the future health of the children they help create?

Answering this question requires long-term follow-up of children born from assisted reproductive technology (ART). While many such studies have been performed evaluating the physical and mental progress of children born from IVF, egg freezing and vitrification are newer technologies.  A new study published in Reproductive Biomedicine Online—“Six-year follow up of children born from vitrified oocytes”—is the first to look at the way children born from previously vitrified eggs develop over their first six years of life.

Background information

What is vitrification?

Vitrification is a method of cryopreservation that, today, is commonly used for freezing eggs and embryos. It’s a “flash freezing” method that cools cells almost instantly to -196º Celsius (or about -320º Fahrenheit), the point at which all biological processes pause.

Prior to the advent of vitrification, embryologists—fertility lab scientists—used a process called slow freezing to preserve eggs. In that process, the egg cell is cooled very slowly—at a decrease of .3º–2º Celsius per minute—until it reaches the final storing temperature of -196ºC.

While the slow freezing technique takes hours, vitrification is completed in minutes. Speed is very important for the success of egg freezing. Studies have demonstrated that the longer the freezing process takes, the more likely it is that ice crystals will form in the cell. Ice crystals can cause damage to cell structures, reducing the chance that the cell will be functional when it’s thawed. Vitrification decreases the risk that ice crystals will form inside the cell. This is especially important for egg freezing, since eggs (as opposed to other things we might freeze, like sperm) are large size cells with a high content of water.

A 2009 study found that vitrified eggs had an over 91% freeze-thaw survival rate (compared to the 61% survival rate of eggs thawed after slow freezing). Today, vitrification is the gold standard for egg freezing, and it’s what the Extend Fertility lab was built around.

What we already know about babies born from IVF

Generally speaking, experts believe that people conceived from IVF are just as healthy as their naturally conceived counterparts, and that any small differences between the populations can be explained by accounting for parental factors, as opposed to by the process of IVF itself.

The same researchers that performed the study we’re looking at today conducted a follow-up study of the mental and physical development of children born from IVF in 2004. They found that, compared to children conceived naturally, singleton babies born after IVF treatments had comparable rates of achievement in specific developmental milestones, such as walking, talking, and emotional development. Singleton IVF babies tended to be smaller than naturally conceived babies at birth and for the first 3–4 months, but that difference leveled out at 6–12 months. Overall, the researchers concluded that that “there was no significant difference between the singletons alone and naturally conceived children irrespective of the ART method.”

These researchers found that IVF babies born in multiples, such as twins or triplets, did have significantly delayed physical and mental development in the first 6 months of life. However, this is likely due to the known developmental differences between multiples and singletons—potentially due to their lower birth weights—and not due to the technique used to conceive the children. Additionally, the developmental progress of all children equalized after 6 months of age.

An even longer-term follow-up study was published in 2017. In that study, 253 teenagers and young adults conceived from IVF and born between 1982 and 1993 were evaluated. No differences were found between the physical and mental health or cognitive ability of ART-conceived adolescents, compared with the reference group.

What we already know about babies born from vitrified eggs

A 2008 study examined the obstetric and perinatal (after birth) outcomes of babies born from eggs frozen with vitrification, then a very new technology in fertility medicine. Researchers looked at the rates of low birth weight, preterm delivery, and congenital anomalies among babies born from IVF with previously vitrified eggs, compared to the averages in the general population. They concluded that “pregnancies and infants conceived following egg vitrification are not associated with increased risk of adverse obstetric and perinatal outcomes.”

The analysis

In this recent study, researchers sent questionnaires to parents of singleton babies born from vitrified eggs at regular intervals over their first six years of life. These questionnaires asked about the babies’ and children’s physical metrics, such as weight and height, as well as their developmental milestones, such as the ability to hold their heads up, eat solid food, sit up, walk, speak, respond to their own names, draw, use the toilet, and socialize. The questionnaires were based on the Japanese government’s Maternal and Child Health Handbook, which outlines what appropriate development looks like for each age group.

They then compared parents’ responses to national averages. First, when looking at outcomes such as low birth weight and chance of preterm birth, their findings reflected the earlier research done on obstetric and perinatal outcomes of babies born from vitrified eggs—egg vitrification is not associated a significantly increased risk of low birth weight, preterm birth, or babies being too small (or too large) for their gestational age.

They also found that, overall, babies and children born from vitrified eggs had about a 90% achievement rate in meeting the developmental milestones. The percentage of children meeting the milestones also increased as they children got older; 100% of six-year-olds from vitrified egg parents met all developmental milestones.

Physical and mental development of children born from vitrified (frozen) eggs

A sampling of the survey results:

Age	Milestone	% of babies born from vitrified eggs who met milestone
3 months	Hold his/her head up	90%
	Eat soup or vegetables	16%
6 months	Sit up	88%
	React to TV or radio	100%
9 months	Crawl	84%
	Play alone	97%
12 months (1 year)	Walk with support	95%
	Understand simple words such as “come here”	100%
18 months	Walk alone	100%
	Say a meaningful word, such as “mom”	97%
24 months (2 years)	Imitate adult gestures	100%
	Run	100%
36 months (3 years)	Draw circles with crayons	100%
	Regularly chewing and eating well	100%
48 months (4 years)	Playing pretend with friends	100%
	Go pee alone	100%
60 months (5 years)	Tell the difference between colors: red, blue, green, and yellow	100%
	Speak clearly	94%
72 months (6 years)	Put on and take off clothes alone	100%
	Read and write names	100%

Bottom line

These researchers concluded that the mental and physical development of children [in this study] were comparable to the data reported by the government. Further study is warranted, since the sample size here was fairly small, and since fertility and obstetric science is changing and refining all the time.

But this research provides another reassuring data point for women considering egg freezing that they’re making a decision that will benefit their future families—whenever they’re ready to start them.

Learn more about egg freezing.

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