Weed with Small Pink Flowers
This appears to be a type of Smartweed (Persicaria, formerly Polygonum; it has several other common names); some are native and others are invasive, and we cannot see enough detail here to tell which this may be. The link below provides a list of photo galleries of species found in MD; Oriental Smartweed is a common weed among them. In either case, those that tend to occur in yards favor moist soils, so it's possible the soil in those areas is staying too wet or compacted for the lawn to thrive and out-compete the Smartweed. Physical removal will be the most immediately effective method of control, especially now, before those flowers ripen and disperse seed. As annuals, they will die this winter, though a pre-emergent herbicide for broadleaf weeds may be useful in spring to prevent their recurrence if the grass hasn't filled-in well by then.