If that 'are you really pregnant this time' rumour is true, then they couldn't have gone down the surrogate route.
That can be explained this way:
Smegs pretended to be pregnant to make Sparry get engaged to her/marry her in Botswana. They went to HMTLQ and told her they were pregnant/engaged/married, she said "I suppose I'll have to say yes". There was a mythcarriage at some point.
Maybe a second "megnancy" around the time the whole thing with her father was going down (or maybe something else when she was about to be exposed/Sparry was getting cold feet). A false positive this time.
Hence, "are you sure you're pregnant this time".
At this point (she pretends) he realises that she is mentally unwell from her mythcarriage and "press hounding" and/or it would be internationally embarrassing if the story of the fake pregnancy gets out - maybe she alluded that his "manhood" would take a hit. So he agrees to get a surrogate, whom Smegs has already lined up.
The big problem with this is that the Gruesomes were either apparently seen drinking heavily on the same tour, or she had turned up hungover/drunk at an event - I'm not sure which it was, but there was drinking involved.
Also, the timeline won't make sense since Smegs was already faking a tiny bump, so a surrogate couldn't get pregnant immediately and with Farkle coming out when he supposedly did.
There were also reports that the Gruesomes had gone to Canada together to retrieve her frozen eggs before the wedding (IIRC). Likely they were already looking at surrogates. I'm not sure Sparry would have married her, roast chicken and evil brain or not, if there was a possibility she wouldn't give him children, unless he didn't mind going the surrogate route.
So likely she had a megnancy to get engaged, had a mythcarriage, they looked at and finalised surrogate(s) - would be sooo romantic (and a proof of his virility, and a way to ensure that attention remained on them) to get pregnant soon after his marriage, quicker than his brother and SIL did, then Smegs faked false positives every time she was about to get caught/Sparry was looking to cheat or didn't do what she wanted. Also a good way to isolate him from the friends who were trying to wake him up. I've heard of women who would do that - it's the less expensive and stressful version of having a baby to "save the marriage".