No, the Red Cabbage test is very unlikely to be a reliable way of determining the sex of a fetus.
Although I was unable to find any reliable, scientific test of the "Red cabbage" method there has been informal study of a related test, the Drano Test. The Drano Test would likely rely on the same method of operation (Colour of mum's urine affecting the colour of another liquid in such a way as to expose the sex of the fetus).
This letter was written to the Journal of the American Medical Association in 1982.
To the Editor
During the past several years, we have been asked
frequently to do the "Drano test" to determine the sex of an unborn
baby. It has been published in the lay press that this is a reliable
means of sex determination. A Medline search failed to reveal anything
in the medical literature concerning the Drano test. As a result, we
performed the test in 100 consecutive pregnant women, checking monthly
during the last trimester. The test was done by adding a small amount
of crystal Drano to approximately 2 mL of urine, agitating, and
interpreting results in one minute's time. Reportedly, the color green
indicates a male baby, and yellow to amber indicates a female. Of the
100 patients, 21 failed to have the same color change consistently. Of
the babies born to these 21, eleven were girls and ten were boys. Of
the remaining 79, we were right in sex determination of 37; of these,
there were 20 girls and 17 boys. We were wrong in 42 predictions; of
these, there were 22 girls and 20 boys. From this brief study, it
would appear that the Drano test for antenatal sex determination is
roughly equivalent to flipping a coin.
Robert M. Fowler, MD
University of Wyoming
You have attributed the changing colour of the Red Cabbage solution to different pH levels of the Urine, which may or may not be true. This medical article on Urine pH attributes high Urine pH to
- Gastric suction
- Kidney failure
- Kidney tubular acidosis
- Urinary tract infection
And low Urine pH to
- Diabetic ketoacidosis
None of which sound like symptoms of pregnancy, boy or girl!
According to this article on Medicine.Net there exists a few ways to predict the sex of a fetus, with varying levels of reliability as well as possible side effects (eg, miscarriage).