Is a One-Week Size Difference in IVF Twins Normal?

Is a one-week size difference between IVF twins considered normal?

Hello, I previously had two D3 frozen embryos implanted and am currently 9 weeks pregnant.
However, since 5 weeks and 4 days, during the vaginal ultrasound monitoring, one embryo has been larger than the other by one week, although both have heartbeats.
Recently, at 9 weeks, the vaginal ultrasound showed the same one-week difference, with one heartbeat at 178 bpm and a crown-rump length (CRL) of 2.23 cm, while the other has a heartbeat of 165 bpm and a CRL of 1.73 cm.
I would like to know if a one-week difference in size among IVF twins is within the normal range, or if frozen embryos of the same gestational age should be similar in size.
Additionally, is there a possibility that the smaller embryo is a growth-restricted fetus? Thank you.

Long & Feng, 30~39 year old female. Ask Date: 2016/04/18

Dr. Cao Guotao reply Obstetrics and Gynecology

Hello, in vitro fertilization typically indicates the possibility of fraternal twins.
Identical twins are more likely to present with a size discrepancy, where one may be larger than the other.
You should monitor your current condition.
Wishing you good health.

Reply Date: 2016/04/18

More Info

When it comes to the development of twins conceived through in vitro fertilization (IVF), it is not uncommon to observe a size difference between the two embryos. In your case, with one embryo measuring 2.23 cm (CRL) and the other measuring 1.73 cm at 9 weeks of gestation, a one-week size difference can be considered within the normal range, especially in the context of twin pregnancies.

Twins can be classified into two categories: monozygotic (identical) and dizygotic (fraternal). In your case, since you mentioned that two D3 frozen embryos were implanted, it is likely that you are dealing with dizygotic twins. This means that each embryo develops independently, and variations in growth rates can occur due to several factors, including genetic differences, placental sharing, and individual embryo viability.

The fact that both embryos have detectable heartbeats is a positive sign. Heartbeat detection is a crucial milestone in early pregnancy, indicating that the embryos are developing and receiving adequate blood supply. While it is ideal for twins to be of similar size, it is not unusual for one twin to be slightly larger or smaller than the other, particularly in the early stages of pregnancy. The growth of embryos can be influenced by various factors, including maternal health, nutrition, and the specific characteristics of each embryo.

Regarding your concern about the smaller embryo potentially being a growth-restricted fetus, it is essential to monitor the situation closely. Growth restriction can occur in some pregnancies, but it is not definitive based solely on size differences at this early stage. Regular ultrasounds and follow-up appointments with your healthcare provider will help assess the growth patterns of both embryos. If the smaller embryo continues to grow, even if at a slower rate, it may still be a viable pregnancy.

In summary, a one-week size difference in IVF twins is generally considered normal, especially in dizygotic pregnancies. Continuous monitoring through ultrasounds will provide more information about the growth and health of both embryos. If there are any concerns about growth restriction or other complications, your healthcare provider will be able to guide you through the necessary evaluations and interventions. It is crucial to maintain open communication with your healthcare team and attend all scheduled appointments to ensure the best possible outcomes for both you and your babies.

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