Question: Does the IUD cause breakthrough bleeding?

Breakthrough bleeding

Breakthrough bleeding (BTB) is any of various forms of vaginal bleeding,[1] usually referring to mid-cycle bleeding in users of combined oral contraceptives,[1] as attributed to insufficient estrogens.[1] It may also occur with other hormonal contraceptives. Sometimes, breakthrough bleeding is classified as abnormal and thereby as a form of metrorrhagia,[2] and sometimes it is classified as not abnormal.[1]
In the context of hemophilia, the term describes a bleeding that occurs while a patient is on prophylaxis.[3]
Presentation
The bleeding is usually light, often referred to as "spotting," though a few people may experience heavier bleeding.
Many people find that the breakthrough bleeding ceases after one or two cycles.
Mechanism
Breakthrough bleeding is most commonly caused by an excessively thick endometrium (uterine lining). This is not a dangerous condition, though the unpredictable and often lengthy periods of bleeding are unpleasant. Breakthrough bleeding may also be caused by hormonal effects of ovulation. Breakthrough bleeding may also itself be a symptom of pregnancy.
Breakthrough bleeding is most common when a woman first begins taking oral contraceptives, or changes from one particular oral contraceptive to another, though it is possible for breakthrough bleeding to happen at any time. Smokers are especially prone to breakthrough bleeding while taking oral contraceptives; though many users experience breakthrough bleeding in the first three cycles of taking the pill, non-smokers tend to see the bleeding dissipate more quickly than smokers.
Breakthrough bleeding is likely due to hormonal fluctuations.  The body is programmed to make certain estrogen levels each day and the estrogen (and some additional hormones, such as FSH, LH, and Progesterone) are responsible for regulating endometrium shedding. Therefore, when new levels of hormones enter the body through oral contraceptives, the body is provided with two ways to receive estrogen.  These excess estrogen levels can cause pre-period bleeding (bleeding through).  This should be regulated in several months.
According to Lange Gynecology and Obstetrics, 8th edition, the most common side effect associated with OC use is breakthrough bleeding. It usually occurs during the first one or two cycles and resolves itself spontaneously. Another common problem is amenorrhea. Persistent break through bleeding and amenorrhea commonly reflect an atrophic, or thin and poorly developed, endometrium.
Use of combined estrogen and progesterone eliminates the normal endogenous hormonal cycling and gradually produces atrophy of the endometrial glands. This is because the dosage of estrogen in the OCs pills is much lower than the quantity produced naturally by the ovaries. Higher quantities produced by the ovaries induce proliferation, but low levels supplied by the pills produce atrophy but are sufficient to inhibit the endogenous secretion of the gonadotropins.
The exact chain of events that lead from an atrophic endometrium to the spotting between menses is not explained by the text. This condition may be corrected by using a pill with a higher estrogen (which will stimulate further proliferation of the endometrium) or lower progestin content (which will reduce its stability).[4]
Treatment
Breakthrough bleeding that does not resolve on its own is a common reason for women to switch to different pill formulations, or to switch to a non-hormonal method of birth control.
See also
Vaginal bleeding
Withdrawal bleeding