With a pregnancy slot, you have the right to use the donor for your treatments (if straws are available). A pregnancy slot also gives you the right to use the donor for future siblings. With pregnancy slots, we ensure that the national pregnancy limits are respected. 
You only need to buy one pregnancy slot – if you do not become pregnant and need to change donor, you can transfer your pregnancy slot. If you do not become pregnant, you can also get a refund for your purchase of a pregnancy slot. 

You can read more about it here: Pregnancy Slot