Even extremely low levels of microwaves can cause miscarriage, altered sex ratios, birth defects, and other effects on reproduction
Ouellet-Hellstrom and Stewart (1993) did a case-control study of over 6600 pregnancies among female members of the American Physical Therapy Association. Those who administered microwave diathermy in the six months prior to or during their pregnancy had more than three times as many early miscarriages as unexposed therapists. The risk increased with increasing numbers of exposures. 
Huai (1979) found abnormal menstruation three times as often in microwave-exposed workers as in unexposed workers. 
The ongoing study in Latvia has found up to 25% fewer boys in certain school grades in the area that has been exposed to the radar since 1971 (Kolodynski and Kolodynska, 1996). 
Navakatikian and Tomashevskaya (1994) found a decrease in testosterone in rats exposed to pulsed or continuous 2450 MHz waves at an intensity of 100 μW / cm2. They review a study by Mikolaichyk which found changes in FSH and LH in the hypothalamus of rats from a single exposure to 10 μW / cm2. 
Krueger and Giarola (1975) exposed laying hens to 260 MHz waves for 16 weeks at an intensity of 5-125 μW / cm2. Egg production was 20% less, a greater percentage of females were hatched, and egg shell quality deteriorated. 
Bigu Del Blanco et al. (1973) found a 14% increase in egg production by hens exposed to continuous 7 GHz waves at 1-400 μW / cm2. The mortality rate of the irradiated chickens also doubled. 
Kondra et al. (1970) found that 6 GHz continuous waves stimulated ovulation in hens at an intensity of 0.02 pW / cm2 (0.00000002 .μW / cm2 ! ). Hens that were so treated from birth showed significantly higher egg production during their egg-laying life, and significantly lower egg weight than the untreated birds. This experiment was designed to simulate the exposure at ground level to the Canadian population from a typical microwave relay tower. It was conducted in Manitoba in the late 1960s. Most places on earth have higher ambient microwave levels than that now. 
A later experiment by the same authors (Kondra et al., 1972) did not appear to confirm these findings, but an examination of the data reveals that the chicks in the second experiment were kept in the light 24 hours a day for the first three weeks of their lives, and that continuous lighting stimulated ovulation to approximately the same extent as the very low levels of micro-waves. 
These experiments are food for thought for anyone who wonders why twentieth century human females are ovulating at ever earlier ages.
Tofani et al. (1986) exposed pregnant rats to 27.12 MHz continuous waves at an intensity of 100 μW / cm2. Half of the pregnancies miscarried before the twentieth day of gestation, compared to only a 6% miscarriage rate in unexposed controls. 38% of the viable fetuses had incomplete skull formation, compared to less than 6% of the controls. There was also a change in the sex ratio, with more males born to rats that had been irradiated from the time of conception. 
Ilchevich and Gorodetskaya report that 10 μW / cm2 decreased litter size in mice and increased the number of stillborns (McRee, 1980). 
Gordon (1973) reviews other similar research in the former Soviet Union. 
 Ouellet-Hellstrom, R. and Stewart, W.F. (1993). Miscarriages among female physical therapists who report using radio- and microwave-frequency electromagnetic radiation. American Journal of Epidemiology 138: 775-786.
 Huai, C. (1981). Assessment of health hazard and standard promulgation in China. Biological Effects and Dosimetry of Non-ionizing Radiation, NATO Conference, Erice, Italy, pp. 627-644.
 Kolodynski, A.A. and Kolodynska, V.V. (1996). Motor and psycho-logical functions of school children living in the area of the Skrunda Radio Location Station in Latvia. The Science of the Total Environment 180:87-93.
 Navakatikian, M.A. and Tomashevskaya, L.A. (1994). Phasic behavioral and endocrine effects of microwaves of nonthermal intensity. In Biological Effects of Electric and Magnetic Fields, D.O. Carpenter and S. Ayrapetyan, eds., Academic Press, N.Y. 1994, pp. 333-342.
 Bigu Del Blanco, J., Romero-Sierra, C. and Tanner, J.A. (1973). Radiofrequency fields: a new biological factor. 1973 IEEE International Electromagnetic Compatibility Symposium Record, New York, June 20-22, 1973, pp. 54-59.
 Kondra, P.A., Smith, W.K., Hodgson, G.C., Bragg, D.B., Gavora, J., Hamid, M.A.K. and Boulanger, R.J. (1970). Growth and reproduction of chickens subjected to microwave radiation. Canadian Journal of Animal Science 50:639-644.
 Kondra, RA., Hamid, M.A. and Hodgson, G.C. (1972). Effects of microwave radiation on growth and reproduction of the stocks of chickens. Canadian Journal of Animal Science 52:317-320.
 Tofani, S., Agnesod, G., Ossola, P., Ferrini, S. and Bussi, R. (1986). Effects of continuous low-level exposure to radio-frequency radiation on intrauterine development in rats. Health Physics 51(4):489-499.
 Gordon, Z.V., Roscin, A.V. and Byckov, M.S. (1973). Main directions and results of research conducted in the USSR on the biologic effects of microwaves. Biologic Effects and Health Hazards of Microwave Radiation: Proceedings of an International Symposium, Warsaw, 15-18 Oct., 1973, P. Czerski et al., eds., pp. 22-35.