It is best to allow a dog to mature prior to spay/neuter. You want the growth plates to be closed prior to spay/neuter to prevent joint problems in the future. For the Labrador Retriever and dogs their size or larger it is best to neuter males at 18-24 months and spay females after their first heat cycle (if they cycle at 6 months than wait for the 18-24 months to be sure growth plates are closed).
Going thru a first heat cycle is not as difficult as people think. Dogs do not bleed like humans do during a cycle. Buy doggy diapers and human sanitary pads to help keep your floors clean. Usually the amount of bleeding is a drop on the floor the size of a pencil eraser.
I suggest you buy 2 pair of diapers because I guarantee there will come a time that you will take your girl out for a potty break and forget to take off the diaper and she will squat and pee in it. This way you have a back up. I also do not put my diapers in the dryer to prevent shrinking.
It is very important to keep the dog on leash at all times when outside, even in your own yard. This is for 4 weeks from the onset of bleeding or swelling of the vulva (most dogs swell first but some bleed first). Intact male dogs can smell a female in heat from miles away and will jump or dig under a fence and are capable of breeding thru a chain link fence. so never leave a female in heat unattended in your yard.
Here is a good article that explains the pros and cons of early spay/neuter.
Please don’t let your vet push early spay/neuter for the health of your dog. This is also important for a female pup that has recessed vulva. See my blog on RV: