REVIEW OF LITERATURE

Michael Kadish

Studies have been conducted recently that suggest there is a definite correlation between colder weather and an increase in the birth rate. While an increase in the birth rate nine months after a winter storm has been a standing joke for years, little scientific research has ever been offered to explain fully this issue.

In order to show the process of an increase of the birth rate, examine each step of the reproduction process must be examined to determine which, if any, of these areas increase in colder weather. A preliminary step is to explain the following terminology.

Added together these comprise a formula describing the probability of conception. See formula #1.

N is the number of coital acts during the cycle. (PPr(c|N=n) is the conditional probability of a conception given n coital acts and is the "biological" component of fecundability. (Lam, 1994. 323.) Furthermore, it should be noted that this is only biological, not psychological. This formula does not examine the amount of time, the precautions, or most importantly, the male factors. The effect of temperature on the sperm will then be analyzed first.

A definite pattern exists throughout the world in seasonal births. On TABLE #2 average statistics are shown supporting the researcher's theory. The most successful string of months for conception is from September through February (Complete Almanac, 1995. p 672). Obviously, these are the fall and winter months, when the weather is getting colder. Studies show that the slightest rise in temperature will slow sperm mobility and sperm count. (Spira, 1991. pp 49-56). Random sperm counts taken in a large national test showed that the summer can shave eleven percent off the mean sperm count. Winter conceptions increase approximately eight percent. (Lam, 1994. p.18). This can be justified by realizing that non-hibernating mammals' blood pressure and pulse rate increase almost to five-fourths as an immediate reaction to the cold weather. Sperm counts increase in similar proportions from summer to winter as well.

While there is definite scientific proof of the male's sperm count increasing in cold weather, one might wonder why the percentage of increase of sperm is so much greater than the increase in pregnancies. One explanation for the discrepancy is photoperiod. While the statistics shown in Table #2 are true for most of the northern hemisphere, the closer to the Arctic Circle, the pattern begins to change. For example, chart #3 shows the statistics in England. (Rotberg, 1986. p.208). In it is a perfect example of photoperiod. As the days get shorter in the winter, less sunlight is received by the eye. The more north one travels, the more drastic the amount of difference. In Alaska during its near total darkness for winter, the suicide rate is phenomenal. As less light hits the retinas than normal, many people feel groggy and depressed, not having an urge for many activities, especially sex. (Ross, 1995. p D1).

However, table three does not cancel out table two. Comparing tables two and three may suggest that everything evens out, but this is not the case for three reasons.

Nevertheless, this phenomenon of photoperiod probably accounts for much of the difference between the more dramatic rise in sperm count and the lesser increase in births.

Since the researcher already knew that the pregnancy rate increases in winter, for the rest of his project the researcher had only to determine why each of the variables in table #1 was either increased by winter or remained unaffected by it. The only negative effect was psychological, the aforementioned photoperiod, which affects women to a lesser degree than men (Hyde, 1979. p 277).

The Pc of a woman is the likelihood that during her menstrual cycle she will become pregnant. Again, the lack of heat is a major reason. Extreme heat, or thirty degrees above mean temperature is a rare time for fertility. Statistics show that nine months after a dry summer heat wave, the birth rate can drop to only fifty percent (Cohen, 1977. p 50).

Pe is the probability that the baby who was conceived in the Pc would survive more than one month in the womb. A miscarriage in the first month may not affect a woman's cycle, and she may never know she was pregnant.

One logical theory suggests seasons affect Pc. Cold weather naturally makes people more cautious (Lam, 1991. p 78). As petty as this may seem, this leads to an important factor. In order to determine the number of possible pregnancies, we must get a complete number of women available to be impregnated. Most women who become pregnant, but then miscarry do not become pregnant again for an average of three months (Bongaarts, 1983. p 18). Since larger numbers of women become pregnant in winter, a larger number of women would miscarry early in their pregnancy. Being less likely to conceive for another three months lessens the total number of women possible to impregnate.. This is another reason why the sperm count increase is greater than the increase in birth rate.

Po is the amount of women with cycles in a set amount of time that are fertile. There has been no testing of this area with seasonal changes. Since it has already been determined that more births occur nine months after the winter, it is obvious that many women are fertile during winter. Further research is needed to determine if the Po changes in winter, and if so, why?

PL is the probability of a miscarriage. There is a definite pattern between this occurrence and temperature. See table four (James, 1971. p 71). A fetus conceived in the winter will be in his last trimester in the summer. The warmer air in the summer is much better for mother and the fetus than the dark weather of winter. With a pregnant woman's already fluctuating hormones and feelings, the photoperiod can make her much more depressed. The combination of these emotions can leave the fetus in some unfriendly chemicals to the brain (Bongaarts 1983, p 37). Besides the psychological aspect, there really is not a factor combining PL and the seasons. The only other theory is that the cold weather might be harsh on the baby, giving it a lesser chance to live. The researcher did not take this seriously, knowing that the percentage of difference is not so great that those who are born in the winter have been weeded through a Darwinian process. This belief has been called downright foolhardy by some (Udry, 197. p.79). The fact that this researcher was born in winter has not led him to the belief that he and fellow Aquarians are in no way superior to people born under warmer signs.

Pf is the odds that with one insemination in a woman's fertile period she will become pregnant. This is the final test. This is the variable that is definitely better in the winter. The combination of the definite increase in sperm in the winter and the greater fertility of the woman in winter leaves no doubt as to the validity of the theory.. The Pf variable is unrelated to the negative effect of photoperiod. This variable assumes, of course, that the couple has engaged in a coital act.

David Lam, professor of economics and population studies at the University of Michigan looked at birth and temperature records from 1941 to 1988 for ten states. His research found that above average temperatures in August were associated with below normal birth rates in April. Lower than normal temperatures in winter caused higher than normal birthrates nine months later (Ross, 1995. p. D-1).

No research, however, has been done using Florida statistics. This researcher decided to test the weather and birth statistics for his city of residency, Jacksonville, for the years 1991-1994. For complete results, look at tables five through eight. The researcher's results were similar to Lam's findings.

After considerable study, the researcher is quite aware of the proved fact that the winter produces more pregnancies. He own study of Jacksonville birth and temperature rates confirms previous studies on other areas. He has decided that this is mostly due to the increase in the male's sperm, as opposed to weather related conditions in the female. He came to this conclusion after realizing little testing has been done on the matter since semen is easier to test than eggs.