Not strictly true. Progesterone-only contraceptives may function in this way (quote from Wikipedia follows):
"Mechanism of action
The mechanism of action of progestogen-only contraceptives depends on the
progestogen activity and dose.
[7]
- Very-low-dose progestogen-only contraceptives, such as traditional progestogen-only pills (and subdermal implants Norplant and Jadelle and intrauterine systems Progestasert and Mirena), inconsistently inhibit ovulation in ~50% of cycles and rely mainly on their progestogenic effect of thickening the cervical mucus, thereby reducing sperm viability and penetration.
- Intermediate-dose progestogen-only contraceptives, such as the progestogen-only pill Cerazette (or the subdermal implant Nexplanon), allow some follicular development (part of the steps of ovulation) but much more consistently inhibit ovulation in 97–99% of cycles.[8] The same cervical mucus changes occur as with very-low-dose progestogens.
- High-dose progestogen-only contraceptives, such as the injectables Depo-Provera and Noristerat, completely inhibit follicular development and ovulation. The same cervical mucus changes occur as with very-low-dose and intermediate-dose progestogens.
In
anovulatory cycles using progestogen-only contraceptives, the endometrium is thin and atrophic. If the endometrium were also thin and atrophic during an ovulatory cycle, this could, in theory, interfere with implantation of a blastocyst (embryo)."
What that all adds up to is that the dosage of progesterone affects the
likelihood that it is functioning as an ovulation suppressant or a preventer of implantation. It also shows that the effect is not the same in all women. Women using this form of contraception, who are concerned about it functioning by preventing implantation, would need to monitor the effects on their own body and judge whether or not ovulation is being prevented.