Study of the transfer of nutrients and other substances from an
expectant mother’s body to that of her unborn child is one of the most
challenging areas of biological research. A team of scientists headed by
N. S. MacDonald has used a whole body counter at the University of
California at Los Angeles to study one aspect of this problem by
comparing the concentrations of radioactive materials in newborn
infants, in babies who are born dead, and in tissues of the
mothers-to-be.
In these studies the scintillation crystal was placed directly beneath a
plastic bassinet holding the babies. Twenty-eight infants, 6 to 24 hours
old, were counted for 45 minutes each. The only radionuclide found was
the ever-present potassium-40.
The bodies of seven stillborn babies were counted for at least 10 hours
each. More kinds of radionuclides were found than in the living babies,
although the large counting time may have affected the results.
The same counting techniques were used with placental tissues from
mothers of three of the stillborns. The placenta is the organ that
nourishes an unborn child and through which substances from the mother’s
bloodstream are exchanged with those in the baby’s blood. The graphs in
Figure 17 show data from this experiment and illustrate the method of
interpreting whole body counter data. When the counts per minute at each
band of gamma-ray energy recorded from the placental tissues (b) were
subtracted from corresponding values from the stillborns (a), it was
found (c) that the placentas contained more of the isotopes
ruthenium-103, ruthenium-106, and zirconium-95 than did the babies that
had been nurtured by these placentas. The babies’ bodies contained more
niobium-95 and potassium-40 than the placentas. Niobium-95 is produced
by the radioactive disintegration of zirconium-95. This suggested that
zirconium-95 atoms do not pass readily through the placenta, but, after
they have decayed to niobium-95, they pass into the baby’s bloodstream
easily.
Actually, the gamma-ray energies of zirconium-95 and niobium-95 are so
similar that the counter cannot distinguish between them. The two
isotopes, however, were separated chemically, and whole body spectra
were prepared from samples of the pure elements. The spectrum (d) of
pure zirconium-95 subtracted from that of pure niobium-95 was strikingly
similar to the spectrum of “stillborn baby minus placenta” on the
graphs. Cesium-137 was added to the synthetic spectrum to provide a
reference mark at the 0.660-Mev point. This revealed that the ratio of
cesium-137 to potassium-40 is lower in babies than in adults.
[Illustration: Figure 17 _Results of experiment studying transfer of
nutrients from an expectant mother to her unborn child._]