Two people who have just had a baby have turned from a to a couple; family de facto group; nuclear family dyad; triad primary group; secondary group

The correct answers are dyad; triad. 

Answer 1: A dyad refers to the smallest possible social unit or group that involves a pair of people. The individuals in a dyad might have a romantic partnership, bonded by friendship, work, family ties, etc. In this instance, a couple who is romantically involved with each other and might or might not be married is a dyad. 

Answer 2: A triad refers to a small group of three people who share familial ties,or are bonded together due to friendship, mutual work, family ties, etc. In this instance, where a couple (a dyad) had a baby, they changed from a group of two, to a group of three (a triad), who share familial ties. The sociologist, Georg Simmel coined the terms dyad and triad.