The school of thought is:

Dogs are the only mammal that ovulate immature eggs (takes 2 to 3 days for them to be ready to fertilize) and the only mammal in which pregnancy is progesterone based rather than estrogen based.

General Timing for Natural Breeding – Progesterone units here are ng/mL so you look for: 2 to 2.9 breed in 4 days & 3 to 3.9 breed in 3 days & 4 to 8 breed today or tomorrow & 8 to 15 breed immediately. Most bitches ovulate between a 3ng and 7ng so watch for a 5.0 as your benchmark. The rise from a 3 to 7 progesterone level is believed to generally occur within a 24 hour period. Bitches ovulate at 5.0 but eggs take 2 or 3 days to be ready to be fertilized. Additionally sperm takes 7 to 8 hours to be ready to fertilize an egg. Eggs are believed to live for about 4 days. Sperm lives 5-7 days, but is most effective at fertilizing eggs the first 48 to 72 hours.

MAKE SURE THAT YOU HAVE TWO PROGESTERONE TESTS TO VERIFY A "NORMAL" PROGESTERONE CLIMB AND OVULATION FOR YOUR GIRL. Remember some girls will stall out on you! Use the timing to know when to retest.

AI – 4 days post 2.5 or 2 days post 5.0 – fresh or chilled semen
AI – 5 days post 2.5 or 3 days post 5.0 – frozen semen

You need at least two progesterone readings – one below 5.0 and one at or above. It is advised to get one low progesterone number early in the season so you have a benchmark number to back into if your girl spikes. If you had a 1.0 on Day 6 and you get a 15 on Day 10 you can easily back into where you are in the breeding cycle without stressing.

Progesterone rises very rapidly once a 5.0 is achieved. Some bitches rise equally fast from a 2.0 (know your girl’s pattern –it is suggested to progesterone test in the season prior to desired first breeding to help identify that bitches pattern)

Canine Reference Range (generally):

<1.0 Anestrus/Proestrus

1.0 - 1.9 Associated with LH surge (2 days prior to ovulation)

2.0 - 3.0 Day of the LH spike

3.1 – 4.9 Day after the LH spike

5.0 Ovulation - but eggs not ready to fertilize for 48 to 72 hours – sperm takes 8 hours to be ready to fertilize the egg

10.0 to 15+ Eggs ready to fertilize

ONCE BRED, I use a pregnancy calendar to give me a estimation of when puppies are due and how they are developing