"Babies born at weekends 'have higher death risk'"
As Singal noted, the single highest-risk day to give birth was actually Thursday - not Saturday or Sunday. But other weekdays had a much lower risk (including Tuesday, which was the lowest-risk day).
So, when you look at an average across all of the weekdays vs. all of the weekend days, the weekends appear to be higher-risk - even though one of the weekdays was actually the highest-risk day.
When you're looking at data that is grouped together (whether it's aggregated or averaged), it's important to remember that aggregates and averages can produce misleading results. (We talk about this in our book, when we discuss how the US Electoral College system of "red states" and "blue states" hides the differences from county-to-county.) Just one more thing to keep in mind as a smart consumer of data.