A satisfactory and normal placenta is of course vital if a normal healthy baby is to be produced. The placenta reaches its full maturity at about the 32nd or 34th week of pregnancy. From this time to the end of pregnancy it slowly becomes more inefficient. This is a very gradual process and even at 40 or 42 weeks the placenta is capable of supplying a mature baby with all its requirements.
Complete failure of the placenta to develop will result in abortion at an early stage. Occasionally, however, the placenta grows but fails to mature properly and will produce less hormone than normal. This will have an influence upon not only the foetus but also the entire pregnancy. The size of the uterus will be consistently smaller than it should be for the duration of pregnancy. The mother’s weight gain will be less than is normally expected. The baby itself will develop normally but will be small and there will only be a small amount of amniotic fluid present.
Such a dysmature placenta can only provide a restricted supply of materials and nutrition to the foetus with the result that the foetus develops normally but conserves its resources by growing very slowly. All the organs are properly formed and they mature normally, so that the end result is a normal but small baby known as a ‘small for dates’, or dysmature, baby. The problem with the dysmature baby resolves around the fact that its requirements continue to increase but the placenta is unable to enlarge its capacity. Sooner or later, therefore, a time comes when the placenta is no longer able to supply the basic requirements and the baby becomes short of oxygen and may even die in utero. The doctor and midwife are carefully trained to recognize the signs and symptoms of the dysmature baby so that labour may be induced before the baby comes to any harm.
A second type of placental insufficiency may occur after the placenta has properly developed. This is because its rate of deterioration is most rapid after the 32nd or 34th week of pregnancy. This may be the result of unknown factors but may also occur in the presence of a raised blood pressure, pre-eclampsia, diabetes or after antepartum haemorrhage. When this secondary type of placental insufficiency occurs it results in a slowing down, and later a cessation, of the actual growth of the baby, although the foetal organs continue to mature normally. Here also a stage may be reached where the demands of the foetus exceed the ability of the placenta to supply them so that the foetus suffers shortage of oxygen and, if it remains unrelieved, may eventually die. It should be stressed that in placental insufficiency the foetus is normal, has developed and matured normally, but is of small stature.
The recognition of primary or secondary placental insufficiency is one of the main duties of the doctor and midwife at the antenatal clinic and modern scientific advances have made this easier. Regular estimations of human placental lactogen (H.P.L.) in blood or of oestriol in blood or, usually, in 24-hour urine collections will indicate placental function. One of the advantages of H.P.L. is that it can be measured on a small sample of blood taken at the same time as other tests. Measurement of the baby’s head size by serial ultrasound scans will demonstrate the amount of foetal growth. Other tests may be used, the exact method varying in different hospitals.