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It involves combining the parents’ heights, adjusting for child gender (the Mid-Parent Rule), and dividing by two.  This method derives from a 1948 article by Horace Gray in the journal Child Development, and is still in widespread use today. The margin of error is +/- 4 inches, which may seem like a large amount (consider the difference of ending up either 5'2" or 5'10"), but is indicative of the difficulty of accurately predicting height in the face of so many variables. The method works in either inches or centimeters. For a male child, increase the mother's height by one-twelfth (13/12 of actual height).  For instance, if the father of a female child is 69 inches, multiply by 12, then divide by 13, for an adjusted result of 63.7 inches. Likewise, if the mother of a male child is 63 inches, multiply by 13, then divide by 12, for an adjusted result of 68.3 inches. In the instance of a female child, with a father at 69 inches (adjusted to 63.7) and a mother at 63 inches, the sum is 126.7 inches.  For a male child, the sum is 69 in. (father) + 68.3 in. (adjusted mother) = 137.3 inches. Using the examples in the last step, example 1 (female child) is 126.7 / 2 = 63.4 inches, and example 2 (male child) is 137.3 / 2 = 68.7 inches. Thus, a female child whose father is 69 inches tall and mother is 63 inches tall is predicted to reach 63.4 inches (5'3").  Likewise, a male child with parents the same height is predicted to reach 68.7 inches (5'9").  The formulas: ((69 x 12 / 13) + 63) / 2 = 63.3 [female child]; (69 + (63 x 13 / 12)) / 2 = 68.7 [male child]. There are also websites that will do the calculations for you if you enter the required heights.  Remember that with the range of accuracy in this method, the female child in the example above could reasonably expect to reach between 59.3 and 67.3 inches, based on assorted genetic and environmental factors. There are variations to the Gray Method that skip the 12/13 adjustment and simply add then divide by 2 the parents' heights, then add 3 or 5 inches for a male child or subtract 3 or 5 inches for a female child.  The results end up roughly similar to the original method.  Thus, for a female child, ((69 + 63) / 2) - 3 =  63 inches, or ((69 +63) / 2) -5 = 61 inches. Or, for a male child, ((69 + 63) / 2) + 3 = 69 inches, or ((69 + 63) / 2) + 5 = 71 inches.
Use the Gray Method for unborn children and children up to age four. Measure the heights of the father and mother. For a female child, reduce the father's height by one-thirteenth (12/13 of actual height). Add the adjusted height of one parent to the non-adjusted height of the other. Divide the sum by two to estimate the child's final height. Put it all together. Use simpler variations, if preferred.