Depending on how far her pregnancy was along, if she was pregnant in August 1914, then the baby would have been registered between Q3 1914 and, as you were allowed 6 weeks to register a birth, up to Q3 1915.
Within that period (anywhere in England & Wales)I can only see one possible male birth where the baby was registered and no MMN recorded (as she was likely to be single)
1915 Q2 Blackburn District William Edward Walmsley - mmn blank
and, using the same criteria, possible female births
1915 Q3 Blackburn District, Annie Walmsley - mmn blank
1915 Q1 Kensington District, Renie Margaret Ogle Walmsley - mmn blank
Q1 1915 Chester le Street District, Sarah Walmsley - mmn blank
Given the districts, none of these look likely, as Margaret's address on the pension index card was still that of her parents' in Preston, and there is no indication that she was claiming upkeep for a child.
Possible scenarios (pure conjecture)
1 She had a miscarriage and lost the baby before it was full term.
2 She wasn't pregnant at that time (Aug 1914), but Matthew had been stationed abroad and this was the first time he'd had the opportunity to go to see her and make amends for the child he'd fathered who had died soon after birth in 1913. (if that little Maria 'was' Margaret's which looks quite possible).
This may be one of those things we will never know.
Boo