I have a breeding pair of [large breeds]. Male is going to be 3 in Jan. And female is 2. My female has had 3 litters. The first was on her very first heat by accident when my mom owned her. My mom's stud boxer broke into her kennel and bred her. She had 5 puppies but only 2 survived. Her next two litters were from my stud. The first from my stud she only had 1 puppy. The second litter she just had yesterday and she only had 2 puppies. One big and one small.

My male and female are always together. And Ive seen them tie once, if not, twice a day when she was in heat.

What would be the reason for such small litters? Is it the dog or the bitch? Should I have the dog tested for fertility? Any help would be appreciated.

Poor bitch! Are you trying to kill her? Far too many letters from a bitch who is only just old enough to have a first litter.

I just cannot believe what I am reading tbh

The higher the COI, the smaller the size of the litter (on average).  Inbreeding results in small litters:  https://www.instituteofcaninebiology.org/blog/coi-faqs-understanding-the-coefficient-of-inbreeding?fbclid=IwAR3WeU9035rUxpj2gXQ7yZmRU6uv8A5oJizXMwJ2ngX0sDd4QhjPQRYjQ1o  (Supported by the fact that you got 5 pups in the litter with the boxer sire...and on her first heat.)

And yes, she is far too young to be having had that many litters.  It is also extremely unlikely that you'd happen to own the best choice of male dog for her...

None of this seems very responsible... Are they fully health-tested?

This is just cruelty.

The fairest thing to do for your bitch is not to worry about litter sizes but take action to prevent any further pregnancies. 
U may choose to neuter ,please read the article on neiuter first to help ,  if not then for her welfare she should not produce any further puppies however u choose to prevent that .jmo

> I just cannot believe what I am reading tbh

Unfortunately I can    One of two (male and female litter mates) I sold to a fellow breeder,  went from that breeder to one overseas at around 2 years.  The female was mated pretty much as soon as she arrived overseas and then went on to have four more litters, one each of the following 4 years before she was finally rehomed (again!) into a pet home.  She was bred to 5 different males but according to the then owner 'only threw herself regardless of the stud dog'.   And that apparently, wasn't what the woman wanted.  Frankly I wept.  So she kept mating her in the hope she did get 'what she wanted'?    Quite why she bought her in the first place, I have no idea, other than she had been top Puppy in her breed that year.   Would that we all had a bitch that we wanted for her lines and type, and she always threw herself!!!

Sadly I'd sold these two unendorsed.

So nothing really surprises me.

Where to start!

Ok, accidents happen & I am guessing your bitch was barely 12 months old (if that), on her first litter. You have purposely mated her on her next 2 seasons (as she is only 2). From your post I am assuming you are intending to mate her on her next season..PLEASE DONT!!  Have your bitch spayed, she has done her bit & let her enjoy the rest of her life.

You should be banned from ever owning an animal.

Won't happen poster is in the US

If you are US based then you can't have had all her relevant health tests done because age 2 is the minimum for many tests like hips and elbows.

Reading this because she has had 3 litters and only 2yrs old i would get her spayed now. She isn't a cash cow for you, poor dog

Immaturitty and overbreeding will also affect litter size.

Your bitch is barely old enough to be producing her FIRST LITTER.