Boxer in her first heat

Question
Our 1 yr old female boxer is in her first heat and we've been doing a good job about keeping her and our 3 yr old male separated, but accidents happen and they mated! Should we worry since this is her first heat if she is pregnant?

Answer
I totally understand your dilemma. You basically have two options, assuming she is pregnant: 1) Have the pregnancy terminated or 2) allow her to have the litter. There is some reason for concern due to her age. She is, for the most part, physically mature but not completely; however, emotionally she still has a way to go. Both of these concerns are precisely why it's advisable to wait until a minimum of 18 months prior to breeding & for larger breeds, two years. There is a chance, due to her emotional/mental  immaturity, that she may not make the best mother. It's possible she may not take proper care of the litter or may even be totally disinterested. However, it's just as likely she will instinctively do what's necessary & will be as good a mother as an older dog. I would urge you to discuss this situation in detail with your veterinarian as he's in a far better position to advise you on the best course of action since he not only knows her health history but can also conduct a hands-on examination.
Physically, there's always the possibility, even in a planned breeding & with a dog at an appropriate age, of having a litter of over-sized puppies which may require a C-section. Again, your vet will be your best bet for giving more definitive advice. If you decide to go ahead with the pregnancy, I would urge you to have an X-ray taken at 50+ days gestation so that the fetal skeletons can be seen & your vet should be able to judge whether or not she'll be able to deliver naturally or if a C-section will be necessary.
I would advise you to contact your vet very soon to discuss your options & seek his advice on what he feels would be the best course of action for your dog. Don't feel badly about this happening -- you did your best to keep them separated but the urge to breed is very strong when a female is in estrus and both the female & male will do almost the unthinkable to fulfill this instinct. I've known of a similar situation where the owner kept the female & male in totally separate rooms only to return home one day to find the male had literally gnawed & chewed his way through the door!
If you have any additional questions or concerns please don't hesitate to let me know -- I'll be happy to help in any way possible. I'll be hoping for the best for you & your girl!