Human implantation has become the focus of considerable scientific interest with the advent of in vitro fertilization and other assisted conception techniques. The frustratingly high failure rates associated with these procedures has led to detailed appraisal of possible causes. Without basic knowledge of the physiological processes involved, little progress has been achieved. Investigations have broadly concentrated on the endometrium/uterine environment and blastocyst quality. However, information on both these aspects in the human being is limited. Even in animal reproduction, the mechanisms of implantation are far from understood, although some of the possible factors involved have been studied. By necessity, much of this review extrapolates from animal data.

We cannot talk to the fetus. Our ability to place the fetal condition accurately in context, therefore, lies with indirect evidence and inferred responses. We can examine the fetus. We can do so in a most comprehensive fashion, addressing the salient points of fetal anatomy, both structural and functional. We are able to perform routine evaluation of every system and the methodology is available to assess critical areas such as the heart and the central nervous system in specific detail.

There is an intimate relationship between the kidney and pregnancy. Renal plasma flow increases by 50–70% during a normal pregnancy and the glomerular filtration rate by about 50%.1 These changes commence in the first trimester and fall in the last trimester reaching normal levels within about four weeks postpartum. These physiological changes are accompanied by striking anatomical changes which consist of dilatation of the ureter, pelvis and calyces, together with an increase in renal parenchymal size. The dilatation i s more marked on the right and may appear in the first trimester. At term, 90% of pregnant women show this change.2

Sodium is the major cation in the extracellular fluid volume (ECFV) and as such, is the most important determinant of osmolality and of the volume of this fluid compartment. Hence any alteration in the control of body sodium will be reflected by changes in the ECFV, including the maternal plasma volume. There is no doubt that expansion of the plasma volume is a necessary and desirable event during pregnancy, influencing positively both maternal and fetal outcome. Therefore, studies of sodium balance in pregnancy provide important information relevant to both mother and fetus.

During pregnancy and lactation there are many changes in maternal calcium physiology which maintain homeostasis in the face of greatly altered calcium balance. In the course of fetal growth and development, 30g of calcium is incorporated into the fetus by term, an amount derived wholly from the maternal system. Most of this accumulates in the latter half of pregnancy, representing a net transfer of 200mg calcium/day (5mmoles). The fact that this is not achieved at the expense of the maternal skeleton is testimony to the conservative and protective adjustments that are seen in calcium metabolism in pregnancy. Furthermore, the changes must persist both in the puerperium and later when lactation presents a source of continuing maternal calcium loss to the suckling infant. The calcium content of human breast milk s i 6–9mmols calcium/l, two to three times the maternal serum level. In the course of one week a normal breast-fed at term infant takes two to three litres of milk, containing 10–30mmols of calcium. The maternal daily calcium intake recommended by the World Health Organization s i 1.25g (30mmol) of which only 25% is absorbed. Thus calcium loss from mother to baby is significant and may not be replaced by diet in many parts of the world.

Efficient respiration in a baby relies on adequate anatomical development of the lungs, effective control of respiration, neuromuscular coordination and anatomical and biochemical maturation of the alveoli so that they are stable, thin-walled and in close proximity to the alveolar capillary bed. As well as maturation of lung function, there must also be a major change in pulmonary blood flow at birth to allow perfusion of the capillary bed with deoxygenated blood in order that effective gas exchange can take place. Successful initiation and sustaining of repiration after birth requires all of these things to occur simultaneously otherwise respiratory failure will ensue.