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Plan-B—Does It Prevent Conception or Full-Term Birth?

Cory Doctorow

Numerous studies are showing that Plan B often fails to stop ovulation when given in the fertile phase when pregnancy is likely to occur.

The well-respected Wilcox study (1995) established probabilities of pregnancy from sexual relations on each day of the fertility cycle. According to the Wilcox probabilities, 16 women in the preovulatory group who were found to have ovulated should have then become pregnant due to recent sexual relations. But no established (implanted) pregnancies occurred. What happened to those 16 embryos? We don’t know precisely how they died, but we do know that sometime between the hour they were conceived and a week-10 days later when they should have implanted in the uterine lining, they “disappeared.” The researchers concluded that Plan B was 100% successful in preventing an establish pregnancy when given prior to ovulation.

The women who received Plan B on the day of ovulation or later were expected to have 8.7 children and, in fact, had 8 established pregnancies. The authors concluded that Plan B has 0% effectiveness when given at or after ovulation, but the conclusion that Plan B does not interfere with implantation applies only when it is given on the day of ovulation or later. We don’t know if Plan B interferes with implantation when given before ovulation. What we do know is that all of the embryos expected to be conceived and to implant when Plan B failed to stop ovulation did not survive to become an "established pregnancy."

Further Evidence

Their findings were corroborated in three other studies that used TVUS and hormone levels to determine the ability of Plan B to delay or block ovulation (Massai, 2007; Croxatto, 2004 and Durand, 2010). Ovulation and higher progesterone levels occurred among 74% of women who received Plan B on Day -4. Taking Plan B on days -3 and -2, produced no delay in ovulation and the great majority of women ovulated.

Noé and Croxatto offered no plausible explanation for why the loss of the expected 16 embryos between fertilization and implantation was not due to an abortifacient effect, 


So that is where things stand. Embryos are conceived and die in the first week of their lives and we don’t know exactly by what mechanism these deaths occur. Because Plan B can double the incidence of ectopic pregnancy, due to slowed transport of the embryo through the fallopian tube, one plausible explanation is that embryos may not reach the uterus to begin implantation during the optimal time frame. But whatever the reason, if a hormonal "emergency contraceptive" results in the death of an early embryo, we have to be honest and call it abortifacient. 

Susan E. Wills
is Spirituality Letter of Aleteia’s English edition.


Mozzanega B, Cosmi C.
How do levonorgestrel-only emergency contraceptive pills prevent pregnancy? Some considerations.
Gynecol Endocrinol. 2011 Jun; 27 (6): 439-42.

Peck R, Velez J.
The postovulatory mechanism of action of Plan B.
National Catholic Bioethics Quarterly. Winter, 2013.

Wills S.
New studies show all emergency contraceptives can cause early abortions. The Charlotte Lozier Institute. 2014 Jan.

Noé G, Croxatto HB, Salvatierra AM, Reyes V, Villarroel C, Muñoz C, Morales G, Retamales A.
Contraceptive efficacy of emergency contraception with levonorgestrel given before or after ovulation.
Contraception. 2011 Nov; 84(5):486-92.

Croxatto HB, Brache V, Pavez M, Cochon L, Forcelledo ML, Alvarez F, Massai R, Faundes A, Salvatierra AM.
Pituitary-ovarian function following the standard levonorgestrel emergency contraceptive dose or a single 0.75-mg dose given on the days preceding ovulation.
Contraception. 2004 Dec; 70(6):442-50.

Massai MR, Forcelledo ML, Brache V, Tejada AS, Salvatierra AM, Reyes MV, Alvarez F, Faúndes A, Croxatto HB.
Does meloxicam increase the incidence of anovulation induced by single administration of levonorgestrel in emergency contraception? A pilot study.
Hum Reprod. 2007 Feb; 22(2):434-9.

Durand M, Koistinen R, Chirinos M, Rodríguez JL, Zambrano E, Seppälä M, Larrea, F.
Hormonal evaluation and midcycle detection of intrauterine glycodelin in women treated with levonorgestrel as in emergency contraception.
Contraception. 2010 Dec; 82(6):526-33.


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