Women undergoing in vitro fertilization (IVF) often have surplus embryos that can be frozen for later use. Some studies have found that babies born from frozen embryos tend to be heavier at birth compared to babies from fresh embryos. In this article, we’ll explore the research around frozen embryo transfer (FET) and birth weight to see if frozen embryo babies really are bigger.
Quick Answer
Several studies have found that babies born from frozen embryo transfer tend to have higher birth weights compared to those from fresh embryo transfer. However, the differences are fairly small, usually 100-200g on average. The exact reasons are still unclear but may relate to hormones and the state of the uterus during FET cycles.
Do babies from frozen embryos have higher birth weights?
Multiple studies have investigated birth weights of babies born from frozen versus fresh embryos.
A meta-analysis published in 2017 looked at data from over 5000 births across 24 studies. They found frozen embryo transfer (FET) babies had higher birth weights compared to fresh embryo transfer (average difference = 109g).
Other studies have found similar results:
- A 2018 study of over 4000 singletons born after IVF found FET babies weighed approximately 128g more than fresh embryo babies.
- A 2021 study of 2700 births found FET babies were on average 150g heavier.
- A small 2022 study found FET babies weighed approximately 200g more than fresh embryo babies.
So most well-controlled studies consistently find a small but statistically significant increase of 100-200g in birth weight for FET babies compared to fresh embryo babies.
Are there other differences between fresh and frozen embryo babies?
In addition to birth weight, some studies have found other differences between fresh and frozen embryo babies:
- Gestational age: Some research finds FET babies are born approximately 1 week later on average compared to fresh embryo babies. This may contribute to the increased birth weight.
- Preterm birth: Rates of preterm birth tend to be lower with FET compared to fresh embryo transfer.
- Size for gestational age: Even when gestational age is accounted for, FET babies still tend to be heavier and larger than fresh embryo babies.
- Congenital anomalies: Rates of major congenital anomalies may be slightly higher with frozen embryo transfer.
However, findings are not entirely consistent across studies. More research is still needed to clarify differences between fresh and frozen embryo babies.
Why might frozen embryo babies be bigger?
It’s not entirely clear why frozen embryo babies tend to weigh more at birth. A few possible explanations have been proposed:
- The frozen embryo transfer process avoids high ovarian hormone levels. Exposure to supraphysiologic hormones may impair placentation in fresh cycles.
- The uterine environment may be more optimal during a frozen transfer cycle compared to a fresh stimulated cycle.
- Frozen embryos that survive the freezing/thawing process may be intrinsically more viable.
Essentially, differences in hormones, the uterine environment, and embryo viability between fresh and frozen cycles may all play a role. But more research is needed to clarify the mechanisms.
How big of a difference in birth weight is there?
While studies consistently find frozen embryo babies are heavier, the differences in birth weight tend to be modest:
- Meta-analyses have found average differences of 100-200g between the two groups.
- In individual studies, differences range from around 50g up to around 250g.
For context, the average birth weight for a full-term singleton is around 3300-3500g. So a difference of 100-200g is a relatively small percentage increase.
Additionally, birth weights have a normal distribution in the population. There is significant overlap between the birth weight ranges of the two groups.
So while statistically significant, a 100-200g average difference is unlikely to be clinically meaningful in most cases.
Is there a higher risk of complications from extra birth weight?
Since frozen embryo babies tend to be marginally heavier, could this increase risks of delivery or neonatal complications?
In general, moderate increases in birth weight do not appear to increase clinical risks. Studies find:
- No significant difference in Caesarean section rates between fresh and FET babies.
- No increase in shoulder dystocia or birth injuries in FET babies.
- No difference in NICU admission rates between the groups.
Additionally, the 100-200g average difference is within normal variability. It likely does not cross threshold levels associated with substantially increased risks.
Therefore, while frozen embryo babies are statistically heavier, this small difference is unlikely to notably increase clinical risks in most cases.
Should birth weight influence choice between fresh or frozen transfer?
Given the minor differences in birth weight, should prospective parents consider this when deciding between fresh or frozen embryo transfer?
Overall, birth weight differences should likely not be a major deciding factor.
Some reasons why birth weight increases may not matter clinically:
- Average differences between groups are small (100-200g).
- There is significant overlap in birth weights between groups.
- Moderate increases in birth weight are unlikely to increase delivery/neonatal risks in most cases.
Instead, the choice between fresh and frozen transfer should focus on factors like embryo quality, treatment burden, scheduling, and cumulative success rates.
However, those at higher baseline risk for complications like shoulder dystocia may wish to discuss options with their doctor.
Conclusions
In summary:
- Babies from frozen embryo transfer tend to be slightly heavier compared to fresh embryo transfer, with average differences around 100-200g in most studies.
- The reasons are still unclear but may involve differences in hormones, the uterine environment, and embryo viability between the two procedures.
- While statistically significant, these small average birth weight differences are unlikely to have major clinical impacts in most cases.
- Prospective parents should focus more on factors like embryo quality rather than small variability in birth weight when deciding between fresh or frozen transfer.
Always discuss your particular risk factors and concerns with your IVF doctor when planning treatment.