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Volume 77, 1948-49
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Influence of Season on Human Fertility.

It is well known that Man and most domesticated animals are polyoestrous. The pioneer work of William Rowan and T. H. Bissonnette showed that there exists an inherent rhythm of reproductive activity. Bissonnette (1931), in his experiments on starlings, proved that light and darkness accelerate or inhibit respectively the onset of the oestrous period. Marshall (1910, 1937, 1942) suggested that the existing rhythm of reproductive activity holds good as long as the animal is not moved from its own hemisphere.

It was postulated and experimentally proved by Bissonnette (1933) and Marshall and Bowden (1934) that the light stimulus passes through the retina to the pituitary by nerve paths and thence to the ovaries. The keeping of ferrets in total darkness (Marshall and Bowden, 1934), hooding of ferrets (Bissonnette, 1933) and removal of the pituitary (Hill and Parkes, 1933) delayed or prevented the onset of oestrus in ferrets. Recently, Burckhardt (1947) succeeded in accelerating the onset of oestrus in mares by strong artificial light.

Hammond (1927) says that although cows will breed throughout the year, there exists an optimum period when the interval between calving and a fertile service is shortest. This interval varies in different countries. According to Hammond, temperature and the prevailing feed conditions as well as the requirements of the dairy industry in various countries, also play an important role. Thus in Western England, where pasture is the main feed, the majority of calvings take place in March or April, while in Eastern England, which supplies most of its milk to cities, the majority of calvings take place in autumn. New Zealand presents a special case where, owing to special conditions, the service period is virtually restricted to the three months from October to December.

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Fig. 1—Ratio of multiple to single conceptions in Liverpool (Edwards, 1938).

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It seems of interest to the New Zealand breeder and the research worker to know whether this period coincides with the time of the year most favourable to conception.

Homo sapiens differs from most wild animals by being polyoestrous. So far few investigations have been attempted to find out whether there is any seasonal rhythm in conception rate in man. Mayo Smith (1895) claims that the highest conception rate is during the summer months, but changes in various countries, e.g. in Greece, the highest conception rate occurs in April, in Scandinavia shortly before June. The only recent data on seasonal fertility in man are the figures published by Edwards (1938). By studying the ratio of multiple-conceptions to single-conceptions in Liverpool he arrived at the conclusion that there are two distinct peaks in conceptions during the year; one around mid-February and another from the middle of August to September (Fig. 1). Edwards claims that this graph shows the seasonal effect uninfluenced by artificial birth-control. He suggests that the increasing hours of daylight are responsible for the first peak, occurring in February, while the decreasing hours produce the drop in numbers of conceptions from September onward. The increase in temperature and improved environmental conditions (mid-summer holidays, etc.) are responsible for the more extended effect in the summer peak. The effect of winter environment finally overcomes the light effect in the first months of the year, leading to the very low level of conceptions in April.

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Otago-Southland. Auckland.
Month of Birth. No. of Births Mean Temp. °F. Minutes of daylight No. of Births Mean Temp. °F. Minutes of daylight. Total Births. No of marriages. N. Z. total.
Single Multiple Single Multiple
1938
June 302 2 47.6 701 287 6 53.8 708 605 993
July 355 3 51.3 798 315 2 55.9 778 680 1,177
Aug. 350 7 53.3 890 323 1 58.8 843 689 948
Sept. 318 1 56.5 943 333 5 62.3 881 663 1,916
Oct. 373 3 58.9 922 343 1 64.9 866 724 1,138
Nov. 363 5 58.6 842 338 0 65.2 810 711 1,094
Dec. 1939 334 6 55.7 752 318 2 63.3 745 668 1,067
Jan. 350 8 51.4 653 372 5 59.6 673 748 2,193
Feb. 286 5 45.4 570 290 6 56.1 614 598 946
Mar. 278 5 41.3 522 324 2 53.4 580 616 1,404
April 290 5 40.9 539 324 2 51.5 591 628 1,031
May 280 3 43.5 607 309 1 51.9 641 597 1,004
June 302 6 47.6 701 308 2 53.8 708 626 1,007
July 353 7 51.3 798 337 3 55.9 778 710 1,334
Aug. 357 3 53.3 890 393 9 58.8 843 774 1,102
Sept. 380 2 56.5 943 349 3* 62.3 881 740 2,008
Oct. 366 6* 58.9 922 409 2 64.9 866 792 1,049
Nov. 372 2 58.6 842 359 1 65.2 810 737 1,091
Dec. 357 5 55.7 752 419 3 63.3 745 792 1,144
1940
Jan 343 4 51.4 653 362 2 59.6 673 717 2,116
Feb. 268 1 45.4 570 322 5 56.1 614 602 1,003
Mar. 315 5 41.3 522 368 4 53.4 580 701 1,466
April 319 7 40.9 539 385 2 51.5 591 722 1,084
May 351 5 43.5 607 390 7 51.9 641 765 1,025
June 353 6* 47.6 701 363 3 53.8 708 735 1,736
July 421 1 51.3 798 436 4 55.9 778 867 1,615
Aug. 415 3 53.3 890 443 9 58.8 843 882 1,279
Sept. 376 3 56.5 943 432 8 62.3 881 830 2,507
Oct. 428 2 58.9 922 433 6 64.9 866 877 1,329
Nov. 442 5 58.6 842 443 3 65.2 810 901 1,303
Dec 413 3 55.7 752 441 5 63.3 745 870 2,428
1941
Jan. 400 4 51.4 653 446 6 59.6 673 866 1,289

Figures for mean temperature, duration of daylight and number of marriages refer to the month of conception corresponding to date of birth, e.g., the figures shown with number of births in June, 1938, refer to September, 1937, etc.

[Footnote] † Average of the mean temperature for Dunedin, Invercargill, and Ophir.

[Footnote] * Includes one set of triplets.

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It seemed of interest to compare some data from this country with those from Britain. The figures collected by the Registrar-General and published by the Government Statistician are of no value, as births may be registered up to sixty days from the actual date of birth. We are grateful to the Government Statistician, and in particular to Messrs. Gardner and Foster, for abstracting the original data in terms of month of birth. They include a northerly and a southerly population sample from the Dominion, i.e. the urban area of Auckland and Otago-Southland. The data are limited by the difficulty in abstracting them.

Table 1 gives the original data of single and multiple births of Auckland and Southland-Otago for the period from June, 1937, to January, 1941, inclusive. It may be noted that both centres have an approximately similar number of births. The number of multiple births in both instances was too small to study the ratio of multiple to single-conceptions, so that we had to rely on the study of the number of total conceptions only.

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Fig. 2—Seasonal trends in number of births in the Auckland and Otago-Southland areas over the period June, 1938—December, 1940.

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Fig. 2 illustrates the seasonal trends of both centres. An average period of nine months pregnancy was assumed and the data have been smoothed by using the formula a+2b+c/4. It shows a distinct trend within the two and a-half years under observation, with the lowest conception rate in winter. The number of conceptions per month increases with increasing daylight, and the highest conception-rate takes place about, or slightly after, the maximum of daylight. It is of interest to note that the described trends are rather more pronounced in the Otago-Southland area (45–46° S.) than in Auckland (37° S.). In particular, the depressions expressing the lowest number of conceptions in the Otago-Southland graph are more pronounced than those in the Auckland curve. Fig. 3 shows the conception-rate expressed as percentages of the monthly average of the period investigated. The view is confirmed that with increase of latitude, i.e. more extensive variation in the length of daylight throughout the year, the extent of the seasonal changes in the rate of conceptions becomes more significant.

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Fig. 3—Number of births in Auckland and Otago-Southland (smoothed) expressed as percentages of the monthly average.

Up to this point it has been tacitly assumed that the seasonal rhythm can be explained in terms of variations in the length of daylight. There are, however, other factors which may have some importance, such as the distribution of marriages throughout the year, and seasonal changes in environment such as temperature. It is assumed that the important factor of birth control affects the conception-rate to the same extent throughout the year. An attempt was made to test this assumption by the use of birth statistics for the Maori population, but unfortunately suitable data are not available.

With regard to the monthly number of marriages, no separate figures for Auckland or Otago-Southland are available. Fig. 4 shows a graph of the monthly number of marriages for the whole of the Dominion plotted against the combined number of Auckland and Otago-Southland conceptions. Marriages in New Zealand are correlated with periods of customary holidays, i.e. two marked peaks occur at Christmas time and Easter. The correlation between conception and marriage is r = + .38, i.e. only 14 per cent. of the total variance in the number of conceptions is associated with the number of marriages.

Temperature in New Zealand according to Markham (1942) seems to play a much lesser role in human biology than in England or on the Continent. However, it will be interesting to note that the correlations between the conception-rate and temperature for Auckland is r = + .37 and for Otago-Southland r = + .69, so that only 13 per cent, of the variance in Auckland and as much as 48 per cent, of the variance in conceptions in Otago-Southland is associated with variation in mean monthly temperature.

Finally, the effect of daylight (the increase of which is naturally largely responsible for seasonal fluctuations in temperature) is shown in Fig. 4. The changes in duration of daylight coincide broadly with the variations in conception-rate. The correlation between the number of conceptions and duration of daylight in Auckland is r = + .31, Otago-Southland r = +.62, and combined figures for both areas r = +.48.

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In view of the higher correlation obtained between temperature and conception-rate the correlation with duration of daylight was re-calculated allowing one month lag in conception-rate, i.e. conceptions in January were paired with duration of daylight in December. This improved the correlations to Auckland r = + .38, Otago-Southland r = + .64 and for both areas r = + .53. These results might be anticipated on physiological basis.

Briefly summarising, therefore: (i) There exists a correlation between changes in duration of daylight and the number of conceptions; (ii) the effect of daylight is much stronger in the Otago-Southland area than in Auckland; (iii) the influence of increasing daylight shown with one month lag slightly improves the correlation; (iv) a correlation of the same order exists between mean temperature and number of conceptions; (v) there is a low but significant correlation between marriages and number of conceptions; (vi) as in England, the peak of conception-rate in New Zealand occurs during the summer months.

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Fig. 4—Number of births in Auckland and Otago-Southland combined (smoothed) compared with the monthly distribution of marriages (smoothed) and average daily duration of daylight.

References.

Bissonnette, T. H., 1931. J. Exp. Zool., 58, 281.

—— 1933. Quarterly Review Biol., 8, 201.

Burckhardt, J. A., 1947. J. Agric. Sci., 37, 1, 64.

Edwards, J., 1938. Nature, 142, 3590, 357.

Hammond, J., 1927. The Physiology of Reproduction in The Cow. Cambridge.

Hill, M., and Parkes, A. S., 1933. Proc. Roy. Soc. B., 113, 537.

Markham, Sydney F., 1942. Climate and Energy of Nations. Oxford Univ. Press.

Marshall, F. H. A., 1910. The Physiology of Reproduction. London.

—— Proc. Roy. Soc., London, 122B: 413–428.

—— 1940. Biol. Reviews, 17, 68.

—— and Bowden, F. P., 1934. J. Exp. Biol., 11, 417.

Mayo-Smith, 1895. Statistics and Sociology. New York.