Hey everyone I would love a bit of advise my dog is a small breed, this is her second pregnancy. She mated on the 21st of December and here due date would be today but when the women came out to scan her she said that she was a week behind and gave her the due date of 29th of this month.
I’m really confused into when my dog is due as I’ve never heard of this before.  I’ve always done it from the day they mated. And counted 63 days from them and that would be here due date she has had milk in her nipples for 5 days and her nipples are very swollen she also is laying on her side a lot. And slightly going of her food when should I expect puppies to arrive. I’m really nervous and my aniexty is throw the roof any help would be appreciated

It's impossible to be very accurate with timing unless you ran progesterone or did cytology.  That's not to say that you absolutely should do those things, but if you don't, you have to accept that you won't have an accurate whelping window at all.

A bitch can be successfully mated days before she ovulates, since the semen can live in her until the eggs are ripe - or she can be mated the very last day the eggs are ripe, before they die.  (They are ovulated and then take about 48 hours to ripen before they can be fertilised.)  That gives you a whelping window of about a week.

As for the ultrasound person's info - it depends on how experienced she is and how much you trust her opinion as to whether to put much weight on that.  The majority of people doing an ultrasound are not going to be able to accurately date a litter to within a week, heck, just confirming pregnancy seems to be difficult enough for some(!) - but if you think she is exceptionally experienced and knowledgeable, I guess she could be skilled enough to do that - it is not medically impossible to be able to date relatively accurately.  (Still not as accurately as with progesterone or cytology.)

Was she ovulation tested?  If she wasn't she may have not quite been ready and the eggs fertilised later

No she wasn’t, but she had been milking now for 5 days

As I never bothered with scans () I always went by the day of the first mating, knowing that conception may not take place that exact day.    I let mine go 2 days over, provided they were otherwise fine but knowing my line (secondary inertia), no longer.

Always remembering too that with a small sized litter, sometimes they can just 'sit', growing bigger by the day making a C.Section more likely to be needed.

Just to say thought my girl’s due date was today but when scanned he said the 16th and he was right!

She had two girls textbook but had to have a c-section when no third pup appeared only for her to be too big and died after an hour of the team working on her.