The Corner

Miscarriage Rates

On a more serious note, I did get an email that attempts to answer the questions with which I began this thread: What proportion of apparent miscarriages or spontaneous abortions do not kill early-stage human organisms because no such organisms existed in the first place? Factoring out these cases, what is the natural rate of death for very young human beings?

Here is a letter a doctor sent in response to Andrew Sullivan’s version of the second question.

The statistic you seek is unavailable, as the fates of products of conception are tracked by hormonal changes in the mother, rather than by cell counts. Thus, although the scientific term “zygote” is unequivocally defined and can be applied to human fertility, it is not utilized in clinical medicine in an epidemiologic context.

This difficulty is exploited by Mr. Ramesh Ponnuru in his Corner posts. In reference to your request for information, he argues that “Many miscarriages… result from incomplete or defective fertilizations, and thus do not represent the deaths of embryos…. it seems pretty clear to me that Sullivan’s estimate counts these events in both his numerator as embryonic deaths and in his denominator as conceptions.” He then goes on to amplify your request: “…I think you’d need the answers to these questions: 1) What proportion of miscarriages are the result of failed fertilizations? 2) Leaving these events out, what’s the natural rate of early death? 3) What proportion of induced (deliberate, non-natural) abortions actually kill embryos? 4) Leaving incomplete or defective fertilizations out, what’s the ratio of natural to deliberate embryo-killings in the U.S.?” In another post, he goes on to state that “… the fact remains that Sullivan can’t generate the statistics he cites on the natural death rate of “unborn children”—his precise scientific term, not mine—without counting teratomas and hyatidiform [sic] moles.”

Mr. Ponnuru’s spelling may be faulty, and his terminology may be loose, but he does highlight the nature of the difficulty you will encounter in garnering an adequate answer to your inquiry.

The term ‘miscarriage’ or in more the commonly employed medical parlance a ’spontaneous abortion,’ refers to a pregnancy that ends before the fetus has reached a viable gestational age. The World Health Organization (WHO) defines it as expulsion of an embryo or fetus weighing 500 grams or less from the mother. This typically corresponds to a gestational age of less than 20 weeks, or in more colloquial language, to a loss prior to the late ’second trimester.’

Spontaneous abortion is the most common complication of early pregnancy. The commonly reported statistic is that approximately 10 to 20 percent of clinically recognized pregnancies under 20 weeks of gestation will undergo spontaneous abortion. Losses in ’subclinical’ pregnancies, those not recognized by a missed menstrual period, are also high, occurring in 15 to 25 percent of cases. These rates vary by maternal age, ethnicity, and medical comorbidity.

These descriptive statistics are quoted from longitudinal, observational studies that track changes in maternal hormones, which reflect the pregnant state. In a classic study in which daily urinary human chorionic gonadotropin (hCG) assays were determined, the total rate of pregnancy loss after implantation was approximately 30 percent; 70 percent of these losses, nearly one quarter of all pregnancies in this series, occurred before the pregnancy was detected clinically.

In a more recent population-based series, daily urinary hCG assays were performed in healthy young women years who were attempting to conceive. Of the subsequent conceptions, loss of a preclinical pregnancy occurred in 26 percent, loss of a clinically recognized pregnancy occurred in 8 percent, and a live birth occurred in 64 percent. The small number of remaining outcomes was comprised of induced abortion, ectopic pregnancy, molar pregnancy, and stillbirth.

Mr. Ponnuru brings up medical oddities such as hydatidiform moles, as these can be considered false positives in pregnancy testing: they yield maternal hormonal changes that mimic pregnancy though they have no chance of yielding a viable birth. As noted in the population study conducted above, such molar ‘pregnancies’ were identified and reported separately in statistics yielding spontaneous abortion rates.

His point with regard to the nature of first trimester spontaneous abortions is more on point. One-third of the products of conception from spontaneous abortions are ‘blighted’ or anembryonic: no embryo is found in the gestational sac. In the other two-thirds of cases in which an embryo is found, approximately one-half are dysmorphic, abnormal, stunted and thus would not develop into a fetus. These unviable products likely result from chromosomal abnormalities or maternal exposure to teratogens.

The term ‘miscarriage’ does not technically pertain to fetal losses that supervene after a gestational age is reached when a child is viable outside the womb. This rate is low for structurally and chromosomally intact fetuses and is more dependent on maternal health and well-being and on the availability of physician oversight and modern obstetric care.

Your desire for the specific percentage of human zygotes that eventually emerge from the uterus as infants is a statistic that I believe cannot [be] empirically documented. However, it is clear that birth, the process of fertilization, uterine implantation, and embryonic and fetal development is fraught with peril, and can be colloquially, if not also theologically, be referred to as a ‘miracle.’ (emphases added by RP)

I don’t know if “exploited” is really the right word above, but I thank the doctor for his informative email, including the partial correction to me.

Ramesh Ponnuru — Ramesh Ponnuru is a senior editor for National Review, a columnist for Bloomberg Opinion, a visiting fellow at the American Enterprise Institute, and a senior fellow at the National Review Institute.

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