> Even for them it has a high failure rate.

Care to cite a source?

Here's a run-down of some modern methods:

> Contemporary, evidence-based methods of natural family planning track the various phases of a woman’s cycle each month via observable biomarkers, such as hormone levels (taken from a urine sample gathered first thing in the morning), basal body temperature (taken by mouth first thing in the morning), patterns of cervical mucus (observed during bathroom visits throughout the day), or a combination thereof. You’ll find the studies that produced the following effectiveness rates linked in the descriptions below.

> The Symptothermal Method (STM) combines basal body temperature (BBT) readings with observations of cervical mucus and sometimes that of the cervix itself. The perfect use rate for this method is 0.4%, and the typical use rate is 1.8%.

> The Billings Ovulation Method uses cervical mucus observations to determine when a woman is fertile and when she is infertile. This universal method has been successfully taught to populations with low literacy rates, little access to technology, and even those who are visually impaired. With perfect use, the failure rate is 1%, and with typical use, it’s 10%.

> The Creighton Model is another method which relies on the categorization of observed cervical mucus. Creighton uses color-coded stickers to interpret a woman’s chart. With perfect use, the method’s failure rate is 0.5%. With typical use, the failure rate is 4%.

> The Marquette Model utilizes a ClearBlue Easy Fertility Monitor to track levels of luteinizing hormone (LH) and estrogen in the urine. Variations on the method use cervical mucus or BBT in conjunction with hormone readings. With perfect use, the failure rate is 0.1%; with typical use, it’s 7%. Marquette Method researchers are currently developing protocols for use of the Mira fertility monitor.

> The Two Day Method is a method in which a woman asks herself if she noticed any cervical secretions today or yesterday. If either answer is “yes,” then she understands herself to be fertile. If both answers are no, she understands herself to be infertile. The perfect use failure rate is 4%, while the typical use rate is 14%. Though this method, in particular, sounds simple (and it is!), it is always best to learn any method from a trained instructor.

https://naturalwomanhood.org/topic/effectiveness-rates/