blast increases identical twins?
1 Replies
Patricia Wright - August 13

Dr Smith
I think I read somewhere that there is a slightly higher rate of identical (i.e. monozygotic) twinning with blastocyst transfers.
Is this true? If so, is there any difference between fresh and frozen blasts regarding this effect? And should this info affect the number of blasts one chooses to transfer at at ime?
I was also wondering about something else I read regarding gender and blastocyst transfer. I think it said that boys are the (slightly) more common outcome from fresh cycles and girls from frozen cycles. Again, is this true or is someone distributing poorly researched guesses instead of sound medical information?
Thank you for your time.


Dr Smith - August 15

In my seven years of experience with blastocyst transfer, I have not observed a higher incidence of identicle twins. However, there have been a number of reports in the scientific literature proposing a link between blastocyst transfer and monozygotic (identical) twins (see references below).

1. Monozygotic twins and triplets in association with blastocyst transfer.
J Assist Reprod Genet. 2004 Apr;21(4):103-7.
2. Incidence of monozygotic twinning with blastocyst transfer compared to cleavage-stage transfer.
Fertil Steril. 2003 Mar;79(3):503-6.
3. Increase in the monozygotic twinning rate after intracytoplasmic sperm injection and blastocyst stage embryo transfer.
Fertil Steril. 2002 Jan;77(1):196-8.
4. The potential association between blastocyst transfer and monozygotic twinning.
Fertil Steril. 2001 Jan;75(1):217-8.

I am only aware of one large scale study of pregnancy following transfer of frozen-thawed blastocyst stage embryos (and the study would have to be large due to the relative rareness of identicle twinning). That study reported a rate of 1.6%. This is higher than would be expected in nature, but comparable to fresh blastocyst transfer (see reference below).

High pregnancy rates can be achieved after freezing and thawing human blastocysts.
Fertil Steril. 2004 Nov;82(5):1418-27.

I am only aware of one study that evaluated the possibility of a gender difference following blastocyst transfer. It did not find any difference (see reference below).

Pregnancy outcome after blastocyst transfer as compared to early cleavage stage embryo transfer.
Hum Reprod. 2004 Sep;19(9):2097-102.



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