Comparison between First-Trimester Twins CRL Imply that First Implanted Twin Delays Implantation of the Second Twin - A New Concept Suggested from Twin IVF Pregnancies

Introduction
It is well established in human that one of the twins is prone to be inferior to the other in terms of placentation. Rodent studies indicate that there is an embryo spacing mechanism that is crucial for normal placentation. Lack of this mechanism in the human uterus may result in an adverse effect of one embryo on the implantation and early development of its sibling.

Aim
To establish if there is a difference in the course of implantation and early development between twin IVF embryos reflected by CRL measurements at 6.5-7.5 weeks of pregnancy.

Material and Methods
This is a study on 75 dichorionic IVF twin pregnancies. Only cycles with transfer of two embryos who eventually delivered healthy twins were included. A control group of singleton pregnancies following transfer of single embryo was formed.

Results
We have found a significant difference in the CRL`s at 6.5-7.5 pregnancy weeks. Mean CRL of the bigger embryo was 7.2 ± 2.4 mm and mean CRL of the smaller embryo was 6.2±2.2mm (p<0.001). Mean difference between twins was 12.3 ± 15.6%, range 0-45%. Compared to the control group, there was no difference between the CRL of the singleton (7.1±1.9mm) and the CRL of the bigger twin embryo but the CRL of the smaller twin embryo was significantly smaller (P<0.05).

Conclusions
The difference between the CRL`s of the twins implies that the implantation of one of the twins delays the implantation and early development of the second twin in most IVF twin pregnancies.