lunar month 
noun
1A month measured between successive new moons (roughly 29 1/2 days).Each lunar month starts on the day of the new moon....
- It is the vital data from which the calendrical relationships between sun and moon cycles may be understood - and therefore the most important piece of numerical data after the number of days in a lunar month and the length of the solar year.
- An intercalary month was added every three years to allow for the fact that a solar year has 365.2422 days while 12 lunar months of 29.5306 days contains 354.3672 days.
1.1(In general use) a period of four weeks.I've always wondered, but assumed it was because one week is a quarter of a lunar month....
- A normal full-term pregnancy lasts 40 weeks or 10 lunar months.