Medical abortion in the first trimester: Update

Key Points

Almost one half of all pregnancies are unintended, and about four in 10 unintended pregnancies end in abortion.¹ Many patients are looking for alternatives to surgical aspiration. And in fact, over the last 20 years, medical abortion has emerged as a safe, acceptable alternative to surgery for patients who decide to terminate a pregnancy during the first trimester.

Of course, surgical abortion in the first trimester is also safe and common. For those women who are strongly opposed to an invasive procedure, however, avoiding the risks and invasiveness of surgery and anesthesia are among the chief advantages of medical abortion. In addition, it can be done very early in pregnancy and managed more privately.²

The most widely used medical abortion regimen combines mifepristone (Mifeprex, RU-486), an antiprogestational agent, with misoprostol, a prostaglandin E₁ analogue. Used today in 31 countries, this drug received the Food and Drug Administration's approval in 2000.^3,4

A discussion of second-trimester medical abortion is beyond the scope of this article. We'll focus instead on issues surrounding medical abortion in the first trimester for the ob/gyn generalist.

How does it work?

Mifepristone blocks the action of progesterone by binding to progesterone receptors without activating them.⁸ Because the body requires progesterone for both the development and maintenance of pregnancy prior to the luteal-placental shift, withdrawal of progesterone during early pregnancy leads to decidual necrosis and the detachment of the products of conception.³ In addition, mifepristone induces cervical ripening and promotes uterine contractions by directly increasing the excitability and sensitivity of the myometrial cells to the action of prostaglandins.^9,10

Early research revealed that mifepristone given alone fell short of achieving early pregnancy termination, given its 64% to 85% success rate for pregnancies less than 49 days.^3,11 This led to the development of a protocol for medical abortion that used mifepristone followed by a prostaglandin analogue.^10-12

Mifepristone was found to gain greater effectiveness when a prostaglandin analogue was added, due to the increased sensitization to prostaglandins produced by mifepristone.^10,12,13

Prostaglandins stimulate uterine contractions by binding to receptors on the surface of myometrial cells. Of two such prostaglandin E₁ analogues-misoprostol and gemeprost-used with mifepristone for medical abortion, only misoprostol is used for this purpose in the US. It's inexpensive, readily obtainable in both developed and developing countries, stable at room temperature, and has been studied extensively.