Billings, E. (1988). The Billings method: Controlling fertility without drug or devices. Ballantine.
Cita Chicago (17th ed.)Billings, Evelyn. The Billings Method: Controlling Fertility Without Drug or Devices. Ballantine, 1988.
Cita MLA (9th ed.)Billings, Evelyn. The Billings Method: Controlling Fertility Without Drug or Devices. Ballantine, 1988.
Atenció: Aquestes cites poden no estar 100% correctes.