Treatment of the embryo and the fetus in the first trimester: current status and future prospects.

An effective treatment is now available to prevent the masculinization of female fetuses with congenital adrenal hyperplasia. Some vitamin-responsive inborn errors of metabolism can be treated prenatally by cofactor administration. Maternal phenylketonuria and maternal diabetes mellitus and the prevention of recurrent neural tube defects are also areas where therapeutic advances are being made. It may be possible to carry out chorionic villi sampling before 8 weeks menstrual age if appropriate catheters and guidance systems (probably transvaginal ultrasound) are used. First trimester diagnosis and treatment of fetal cardiac arrhythmias could prove to be very important, as they are later in pregnancy. Future possibilities for progress include gene microinjection into zygotes, classification and treatment of fresh embryos, biopsy and frozen storage of genetically at risk embryos, and therapy of preimplantation embryos by chimera formation or gene introduction by retroviruses.