When we develop the embryo then its quality depends on AMH and Sperm quality. The embryo is tested with either Blastocyst transfer or with PGS/PGD.
After proper testing, these Embryos get stick to the uterus wall and then it's connected with mother. To achieve twins pregnancy we transfer two to three embryo and Pregnancy chances get increases, although we cant assure Surrogate mother will conceive twins.
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( A hope for millions )