In the UK, we either titre (anti-K, or other Kell-related antibodies) or quantify (anti-D and/or anti-c) every four weeks to 28 weeks of gestation, and then every two weeks until delivery. All other antibodies are only monitored from when they were first detected (probably at booking) at 28-weeks of gestation, unless the titre is particularly high.
In all cases, however, if a known antibody specificity is NOT detected in an antibody screen, where, of course, the cognate antigen is expressed, then it is not worth performing the titre at all. It is just a waste of time, money and reagents. In such cases, however, we would perform the antibody screen over the time periods as described above.