Firstly - welcome to the forum
This is an area that causes much confusion and argument especially between vets. There has been much research done into this.
The issues:
If you spay a ***** before her first season the chances of her getting mammary tumours is almost zero. As the ***** has each season the chances of her getting mammary tumours increases, until middle age when there is no protective benefit (a 5 y/o ***** gets no protection).
If you spay a ***** before her first season there is a much higher chance of her suffering urinary incontinence, that will require treatment for life. Having just 1 season significantly reduces the chances of this. (although urinary incontinence is something we see occasionally in spayed bitched)
Around a third of unspayed ******* get a condition called pyometra where the womb becomes infected and full of puss. This requires an emergency spay when the ***** is old and sick and is therefore more expensive and a much higher risk.
My preference is to spay a ***** 3 months after her first season - I see it as a good compromise between reducing the risk of mammary tumours without a great chance of a dog peeing in the house for years!
If you want to know more I will be pleased to ask. Other vets may have different views!