Factors affecting the success of mid-trimester amniocentesis.

Of 298 women who had mid-trimester amniocentesis carried out in a single hospital unit in a 6-yr period, an increasing proportion were carried out for detection of neural tube defects. A clinical decision could be made following the first amniocentesis in 77% of cases. There were 16% where liquor was not obtained at the first attempt, and a further 7% where cell growth or biochemical testing was unsatisfactory. The outcome was influenced by the experience of the operator and timing of procedure. The success of the first procedure was 59% before 16 wk of gestation and 86% between 16 and 20 wk. The incidence of blood-staining of the liquor was higher with an anterior placenta (19%). Despite difficulties, the abortion rate (3.5%) does not appear to be higher than would be expected.