By the time they reach 1 month, sudden changes are becoming visible. The very first you will notice is the raising of head and chest especially when they are at a prone position (lying on stomach). This particular act aids them in strengthening their neck muscles.
The other thing you would soon notice is their stare to your face intently. This signals their social development. From focusing on the details such as your eyes at the early days, now they can already process more the whole such as your face. As much as they love seeing you and your different facial expressions, try showing theirs as well by letting them view in front of the mirror.
In addition to the social development considered also as physical is social smile. You can observe that at the sound of your face. Like how you show your different facial experiences, try to experiment this time your voice. Sometimes they associate voices with food, warmth or comfort.
However the most dramatic is their ability to support now their upper body with their arms when lying on stomach. This is the perfect time to bond and have a particular tummy time to be able to the following physical developments.