In: Fertil. Steril. 1998; 70: S 15- 6

Increased incidence of monozygotic twinning following the transfer of blastocysts in human IVF/ICSI

P.M. Rijnders, H.C. Van Os and C.A.M. Jansen

Dept of IVF, Reinier de Graaf Groep, loc Diaconessenhuis Voorburg, The Netherlands.

Objective: The transfer of blastocysts in human IVF/ICSI is associated with a high implantation and pregnancy rate. We have shown that a selected group of patients may benefit from extending the time of embryo culture to five days, especially those with many embryos of roughly equal quality in which it is hard to choose the best ones for transfer on day 2 or day 3. So far we have achieved more than 100 pregnancies after the transfer of embryos on day 5. However there seemed to be an increase in the incidence of the presence of two fetuses in the same gestational sac in this group.

Design: Pregnancies were analysed for the occurrence of monozygotic twinning in relation to the day of transfer.

Methods: Vital pregnancies in our program between January 1989 and December 1997 (excluding pregnancies from cryopreserved embryos, from unstimulated cycles, and in part, from transport and satellite IVF) were analysed in three groups of patients: those with transfer on day 2, day 3 and day 5 after insemination. In all pregnancies a transvaginal ultrasound had been performed at least three times: at about 6 weeks, at 9 weeks and at 12 weeks gestation. All pregnancies were analysed for the presence of more than one fetus in the same gestational sac or for the occurrence of more sacs than embryos replaced. These cases can only be explained by the phenomenon of monozygotic twinning.

Results: In total 13 monozygotic twins were identified (Table)

Gr	day ET	pregnancies	sacs with FHB	monozygotic twins	% per pregnancy	% per sac+FHB	P
1	day 2	960	1212	5	0.52 %	0.41 %
2	day 3	735	937	5	0.68 %	0.53 %	p = 0.68/ 0.67^a
3	day 5	111	128	3	2.7 %	2.3 %	p = 0.006/ 0.01^b

Table: Number of pregnancies, sacs with at least one heartbeat, monozygotic twins and percentage of monozygotic twins per pregnancy and per sac with ³1 FHB in relation to the day of transfer, ^a = gr1 versus gr2; ^b = gr 3 versus gr 1 and 2 per pregnancy and per sac. There were no differences in age or duration of infertility. Two out of the 3 patients that had monozygotic twinning after replacement of blastocysts had had at least one expanding blastocyst replaced.

Conclusion: There is a significant increase in the rate of monozygotic twinning in relation to the replacement of blastocysts on day 5. This phenomenon has not been reported before in the human in relation to blastocyst transfer. As monozygotic twinning is associated with an increased obstetrical risk this phenomenon must be considered as a drawback and must be taken into account when one contemplates replacement of blastocysts.