Having been a supervisor in another profession but not dissimilar in other respects to aviation, I have been down the "Jessica" road several times. When they first become pregnant their rate of sickness increases because of the fundamental changes their body is undertaking. They then go on to maternity leave. When they are due to return to work, they put in an appearance for a few days and then go off sick with stress caused by trying to work and tend the new born child.
This process can be repeated one or more times depending on the size of the famly they choose.
In the mean time you are short staffed as they cannot be replaced and their work has to be shared out amongst the remaining staff.
If you have two or more similar females working for you the job does not get done.