Both Nicola and Martin said Corrie and April were together in London the weekend before his disappearance (remember the photo on the Shard?). That's when M also said A revealed the pregnancy. Could the text messages have been a continuation of that (alleged) face-to-face conversation? For me, it's a big leap from A merely 'feeling unwell' whilst on holiday to thinking she might be pregnant (maybe they'd realised they'd had a slip-up with contraception, maybe there were particular symptoms). Plus the texts would have been over a very few days between the London trip and the Friday C vanished.
"Then there were the confusing statements about April Oliver, the 21-year-old fitness instructor who was publicly announced as his girlfriend at the beginning of this year. Initially the page said he was single and always insists Corrie didn't know he was going to become a father when he went missing. But Martin said that it 'totally untrue', and revealed Corrie was told during a trip to London with April just six days earlier. 'To say he didn't know is a lie,' Martin said." From:
https://www.google.com/amp/www.dail...rents-war-appeal-missing-Corrie-McKeague.html