Stanford, J. B., & Mikolajczyk, R. T. (2002). Mechanisms of action of intrauterine devices: update and estimation of postfertilization effects. American Journal of Obstetrics and Gynecology, 187(6), 1699-1708. https://doi.org/10.1067/mob.2002.128091
Stanford JB, Mikolajczyk RT. Mechanisms of action of intrauterine devices: update and estimation of postfertilization effects. Am J Obstet Gynecol. 2002;187(6):1699-1708. doi:10.1067/mob.2002.128091
Stanford, Joseph B., and Rafael T. Mikolajczyk. "Mechanisms of action of intrauterine devices: update and estimation of postfertilization effects." American Journal of Obstetrics and Gynecology, vol. 187, no. 6, 2002, pp. 1699-1708.
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IUDs work both before and after fertilization, a model estimates
A review and math model studied how IUDs work. Every IUD type, inert, copper, or hormonal, works both before and after fertilization. The copper IUD acts more before fertilization. Effects after fertilization still matter for every type.
Key Findings
For an inert IUD, the model estimated that 99.1% of fertilized eggs are lost. Inputs were 15.6% fertilization per cycle and 1.9 pregnancies per 100 woman-years.
No direct fertilization data exist for the modern copper IUD, so the model assumed 4.1% and 8.1% per cycle. Those assumptions gave estimated losses of 99.1% and 99.5% of fertilized eggs.
In women having gynecologic surgery, eggs were recovered from 8 of 27 women with copper IUDs (30%) and 64 of 115 women with no IUD (56%).
The model puts losses caused by the IUD at 0.2 to 1.0 per woman per year after fertilization, which the authors call a conservative estimate.
With no effect after fertilization and fertilization in 1% of cycles, the model gives 4.7 pregnancies per 100 woman-years. The modern copper IUD rate used was 0.5 per 100 woman-years.
Interpretation
This review examined IUD studies going back to 1966. The authors built a math model to estimate how each IUD type works before and after fertilization. They call their numbers a conservative estimate. They expect the true effect after fertilization to be even higher. The findings support direct studies that measure fertilization in women using modern copper and hormonal IUDs.
RRM Context
The authors suggest doctors share these estimates with women who object to effects after fertilization. The paper also found that most women with hormonal IUDs still ovulate. Ovulation held true even when their periods stopped or got lighter. Hormone tests and ultrasound proved this. Ovulation needs direct confirmation beyond bleeding patterns. Restorative reproductive medicine reads ovulation directly, using fertility awareness charting and ultrasound.
Our editorial summary of this paper, not the article's abstract.
Abstract
There are many potential mechanisms of action for the intrauterine device (IUD), which vary by type of IUD (inert, copper, or hormonal). This paper reviews the evidence for each potential mechanism of action. On the basis of available data for fertilization rates and clinical pregnancy rates, the relative contribution of mechanisms acting before or after fertilization were quantitatively estimated. These estimates indicate that, although prefertilization effects are more prominent for the copper IUD, both prefertilization and postfertilization mechanisms of action contribute significantly to the effectiveness of all types of intrauterine devices.
