It's hard to say how much impact it would have if you have 2 weeks off in August without knowing how much you earn per week, but I would imagine it would probably only impact the first 6 weeks, as it's unlikely to take your average weekly earnings down below £140.98.
Basically, your SMP is calculated based on weeks 17-25, so you should try to avoid having time off sick then if you can. But your health and the baby's health are the most important.
Here's an example. Say you earn £18k a year. Your weekly earnings are £346.15 before tax. That means for the first 6 weeks you'd receive £311.54, and for the remainder you'd receive £139.58.
If you were on SSP for 2 weeks during that 8 week qualifying period, your average salary would drop to £281.95 (£346.15 (normal pay) x 6 + £89.35 (SSP) x 2, all divided by 8 to get the average). That means for the first 6 weeks you'd receive £253.76, and for the remainder you'd receive £139.58. So only the first 6 weeks would be affected.
But if your average weekly pay for that 8 weeks drops to between £112 - £140.98, all your maternity payments would be reduced. If your average pay dropped below £112 you wouldn't qualify for SMP and would get Maternity Allowance instead.
Again, it's only am issue if you get SSP - if you're paid in full for your sick leave, it won't be impacted at all.