"Limited divisibility of tasks. Adding more people to a highly divisible task such as reaping a field by hand decreases the overall task duration (up to the point where additional workers get in each others way). Some tasks are less divisible; Brooks points out that while it takes one woman nine months to make one baby, "nine women can't make a baby in one month"."

https://en.wikipedia.org/wiki/Brooks%E2%80%99_law