Medical Definition of Pregnancy: "period of time between fertilization of the ovum (conception) and birth, during which mammals carry their developing young in the uterus (see embryo)." See Columbia Encyclopedia entry on Pregnancy

Conception. [L. conception 1. the fecundation [impregnation or fertilization] of the ovum. Dorland's Illustrated Medical Dictionary

See also: NIH Medical Encyclopedia: First trimester pregnancy = "The early stage of pregnancy from conception to 12 weeks gestation or about 14 weeks from the first day of the last normal menstrual period (LNMP)." (emphasis added)

Ectopic Pregnancy: "A pregnancy in which the fertilized egg implants in tissue outside of the uterus and the placenta and fetus begin to develop there..." (cf. Open Directory)

See also: Just a Little Bit Pregnant

Legal Definition of Pregnancy

An 80% Spontaneous Abortion Rate?

(Also: What is the Medical Definition of Pregnancy?)


Dear Fr Bloom,

In re your earlier question, the estimates of an 80% spontaneous "abortion" (although failure to implant isn't really an abortion, but that's a whole different topic) are based on extrapolations from such research as reproduced below in the abstract from BioMedNet. The tabular information at the end doesn't cut and paste well, and I don't know if a non-member can go directly to the abstracts - but membership is free, easy, doesn't generate any spam, and is just as easy to undo (should you wish to visit the site). The search which turned up the below was done on the phrase : human and embryo and survive The extrapolations are (so far as I know) based on the assumption that an implanted embryo is equivalent to a "natural" embryo to an otherwise ready uterus. There is no apparent reason that it shouldn't be, so this is a reasonable assumption.

The abstract: Reprod Nutr Dev • Volume 28 • Issue 6B VIEWING OPTIONS: [MEDLINE], [full MEDLINE], [related records] Please note that the original text is not in English

[Timing of embryo transfer and success of pregnancy in the human] Mandelbaum J, Junca AM, Plachot M, Cohen J, Salat-Baroux J Reprod Nutr Dev 1988 28:6B 1763-71 Vernaclular Title Moment de la transplantation embryonnaire et succès de la gestation chez l'humain. Abstract The best moment for human embryo transfer has not yet been accurately determined. The human uterus is able to receive young embryos (2, 4 cell-stage and even pronucleated eggs), insure their growth and implantation at a rate that does not exceed, however, 15% for one transferred egg. At the present time in vitro culture to the blastocyst stage, which impairs human embryo viability, should be avoided. Contrary to classical IVF, it becomes possible to dissociate embryo and endometrial ages when transferring frozen-thawed eggs. Our study concerns 443 spontaneous, stimulated or artificial cycles, performed in patients with normal or without endogenous ovarian function (tabl. 1). There was trend towards enhanced pregnancy rates (17%) for synchronous as compared to one-day asynchronous transfers (9%) (tabl. 2). Similar data, widely stated in mammals, emphasize the necessity of a precise chronology of embryo transfer. Embryos only survive and get implanted when placed in a receptive uterus. The onset of the refractory period appears to be closely linked to the maternal steroid environment. Consequently, no pregnancies arose from transfer of donated embryos beyond 6 days of progestative supply in women deprived of endogenous ovarian function. In the same way, administration of progesterone 40 h before oocyte recovery seemed to advance the refractory phase. Indeed, a reduction in the pregnancy rate per transfer was observed in such circumstances without any obvious impairment of embryo viability (tabl. 4). The best pregnancy rate was obtained in synchronous transfers while a one-day disynchronization reduced this rate by half.

MeSH Animal, Blastocyst (PH), Embryo Transfer (*), English Abstract, Female, Fertilization in Vitro, Human, Menstrual Cycle, Pregnancy, Time Factors, Uterus (PH) Author Address Service de Gynécologie-Obstétrique, CHI Jean Rostand, Sèvres.


Dear Michael,

Thank you for your e-mail with all the research. I did join the BioMedNet and found it fascinating to look at the different abstracts - altho I wasn't quite ready to put out $25 for any of the full articles, including, I must admit, the one you referred to, especially since my French is pretty limited. I assume you know more about medicine than I, though I don't think either of us would be satisfied by a simple "appeal to authority." In that spirit I do have some questions about the

"assumption that an implanted embryo is equivalent to a 'natural' embryo to an otherwise ready uterus."

Can they duplicate in vitro the environment of the Fallopian tube? Or would things like way ovum is removed, combined w/ sperm, temperature, light, surrounding fluid, means of transport, etc. all be things one could discount? (I gather the study concluded the quicker the transfer is made, the better chance of survival.) Also I didn't get from the abstract whether the study involved couples w/ fertility problems.

You alluded to another question I would like to take up, namely the definition of pregnancy. While surfing BioMedNet I came across this:

Pregnancy Trimester, First [MeSH Record] [Search] Scope Note: Period of pregnancy from the first day of the last normal menstrual period through the completion of 14 weeks (98 days) of gestation. (

It seems the word "pregnancy" is used fluidly. Am I correct in saying the above definition is employed when they refer to a baby's development at, say, the "tenth week of pregnancy"? I noticed in an abstract on "emergency contraception" a different definition of pregnancy was being used. Is this being done to obfuscate non-medical people? A collateral question: are there pregnancy tests which can detect the hormonal changes caused by fertilization?

Any light you could shed, Michael, would be appreciated.

Fr. Phil Bloom


See also Ectopic Pregnancy: A Catholic Perspective

Therapeutic to Reproductive Cloning

The Abortifacient Properties of Emergency Contraception and the Implications for Informed Consent


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