Cristina (Sandra Oh) found out in the last season finale that she is pregnant, but while she does not want to go through with it, Owen is keen to have the baby.
McKidd has now warned that the couple will keep arguing about the situation.
"It's not going to be easy for them," he told E! Online. "It's more than a bump [in the road]... it's kind of a sinkhole, actually.
"They're really hitting it hard. He wants kids, she absolutely does not. It's not going to be pretty."
Reports have suggested that Cristina will make a decision about her pregnancy in the eighth season premiere, but McKidd explained that the storyline will continue after that.
"In the season premiere there will be a resolution to all of that in a quite unexpected way," he teased. "It resolves, and then you think it's put to bed, but it isn't."
Grey's Anatomy will return for its new season with a two-hour premiere on September 22 on ABC.
> 'Grey's Anatomy' Kevin McKidd teases Owen twist
Watch a trailer for the new season of Grey's Anatomy below: