Invasive prenatal diagnosis should only be performed in multiple pregnancies by a specialist in a tertiary level fetal medicine unit who has experience in performing selective termination of pregnancy if required. The uterine contents need to be mapped thoroughly before the procedure is undertaken to ensure that separate samples are taken from each fetus and that each twin can be identified accurately at a later stage. Detailed mapping involves identification of chorionicity, location of individual placental sites and cord insertions, plane of the dividing septum in three dimensions and identifying fetal sex.
Amniocentesis is the preferred option in most units because of the relatively high risk of cross contamination of chorionic tissue with CVS in dichorionic twins (2-6%), leading to false-positive or false-negative results. However, CVS is appropriate in a monochorionic pregnancy if the operator is sure of the chorionicity. Most clinicians use two separate puncture sites when performing amniocen-tesis again to limit the risk of cross contamination although there are series with single entry techniques traversing the placental membranes and low complication rates.
There are no randomized trials to indicate procedure-related loss rates in twins. Background loss rates, however, are appreciably higher. Recent series suggest that total fetal loss rates in twins after amniocentesis (3.5-4.0%) or CVS (2-4%) may not be much higher than background rates. A case-control study of 202 twins undergoing mid-trimester amniocentesis reported a loss rate only 0.3% higher than in control twins .
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Once your pregnancy is over and done with, your baby is happily in your arms, and youre headed back home from the hospital, youll begin to realize that things have only just begun. Over the next few days, weeks, and months, youre going to increasingly notice that your entire life has changed in more ways than you could ever imagine.