Polyhydramnios is defined as a deepest fluid pocket of more than 8 cm or an amniotic fluid index of at least 25 cm or more. Its incidence has been reported to range from 0.4% to 3.3%. Chronic polyhydramnios which develops gradually over weeks or months is more common than acute polyhydramnios.

Various pathways determine production and absorption of amniotic fluid, amongst which are fetal swallowing, fetal urinary production, fetal lung fluid excretion, fluid transfer through the chorionic plate and to a minor extent fluid excretion by the fetal salivary glands. Normal amniotic fluid volumes display a wide distribu­tion with a marked increase up to about 34 weeks and a gradual reduction there­after. Determination of amniotic fluid volume includes single deepest pocket and amniotic fluid index measurement. Abnormal amniotic fluid volumes include both oligohydramnios and polyhydramnios. Whereas oligohydramnios is mostly associated with fetal pathology, polyhydramnios may be due to abnormal fetal or maternal conditions or may be idiopathic. The predictive value of amniotic fluid index and single deepest amniotic fluid pocket for oligo- and polyhydramnios appears to be limited.

The introduction of ultrasound technology represented a unique step forward in our ability to make a diagnosis regarding the human placenta. Rapidly, it became clear that the new technology had far more advantages than x-ray, thermography and scintigraphy in the ability to locate the placenta, and it left the old techniques obsolete. Even the particularly difficult problem of defining the exact delineation of the border in cases where placenta praevia was suspected is now of historical interest only as a consequence of high-resolution ultrasound and the transvaginal scanning approach.

The placenta is regarded as an organ of fetal origin as it develops solely from the outer cell layer of the blastocyst, the trophoblast. The contact with the uter­ine endometrium and the trophoblast induces a proliferation of the trophoblast. Some of the trophoblast cells lose their cell membrane and form a syncytium, the so-called syncytioblast. This process stimulates a decidual reaction in the endo­metrium that causes the stroma to become thicker and highly vascularized, then called decidua. A thin capsule of the decidua called decidua capsularis covers the part of the embryo which is protruding into the endometrial cavity. The decidua at the embryonic pole develops into the decidua basalis, which takes part in the formation of the future placenta. During early development, the vessels support­ing the decidua capsularis regress and the smooth chorion or chorion laeve devel­ops.6 The vessels supporting the decidua basalis are retained and the leafy chorion or chorion frondosum is developed and the growth process of the placenta, which takes most of the remaining time of the pregnancy, then commences (Fig. 6.2).