Tradition and theology
Notions of pregnancy duration date back centuries in the literature. In 'Rígsþula', part of the collection of poems named Den eldre Edda (the elder Edda) (c. 1000 AD), the god Heimdall embarks on a journey (4). He visits three married couples, sleeps with each of the wives, and sires the beginnings of three social classes: slave, farmer and noble. In each case, the woman gives birth nine months after Heimdall's visit.
In medieval folk ballads, pregnancy duration is frequently rendered as 40 weeks (5, 6). The ballad 'Hustru og Mands Moder' (wife and mother-in-law) is known in Norwegian, Danish, Swedish and English variants:
I fyrretyv Uger gik Marri mæ Krist;
å så gör hver en Dankvin for vist.
(7. pp. 412)
[Forty weeks went Mary with Christ;
And so each Danish woman must.]
(English version from https://www.gutenberg.org/cache/epub/44969/pg44969-images.html)
In modern language:
I førti uker gikk Maria med Kristus;
og slik gjør hver en kvinne, det er visst.
[Mary carried Christ for forty weeks;
and so, of course, does every woman.]
In Landstad's collection of folk ballads, we find:
Heime sat hon Kristi fruga
úti mánanne níe,
til deð leið at stundo dei
hon sill' eige sonen friðe.
[At home she sat, Christ's wife
for nine long months,
until the appointed hour arrived,
and she gave birth to her own Son of Peace.]
(8)
In modern language:
Hjemme satt hun, Kristi kvinne,
mens de ni månedene gikk,
til det led mot tiden
da hun skulle eie fredens sønn.
[At home she sat, Christ's woman,
through the nine months,
until the time approached
when she would give birth to the Son of Peace.]
In Christian tradition, Jesus was believed to have been conceived by the Holy Spirit on the Annunciation, 25 March. Drawing on such theological reasoning, the doctor, anatomist and physiologist William Harvey (1578–1657) wrote in the mid-17th century that pregnancy from fertilisation to birth was 275 days; from the Feast of the Annunciation to the birth on 25 December (9). This stance was frequently adopted, including in Norwegian medical literature, well into the 19th century (10, pp. 257–8). The Virgin Mary was regarded as the protector and guardian of pregnant and labouring women, and her pregnancy was considered a model for all others (Figure 1). Her role as a biological exemplar reached its zenith in the 17th century (11).
The duration of pregnancy as a cultural concept is also recognised in other societies, often associated with particularly important numbers (12). In these accounts, the period is measured from intercourse to birth. Pregnancy was traditionally counted as ten lunar months, each comprising four weeks, with each week consisting of seven days. The number 40 holds a special place in culture, religion and tradition, including in connection with other phenomena (1). Comparisons with livestock led to human pregnancy being considered to last nine months. A rhyme from Helsinge on the Danish island of Zealand reads
En Ko og en Kone,
en Pige [dansk uttale: pie] og en Kvie
de gaar lige [dansk uttale: lie] længe
[A cow and a wife,
a girl and a heifer,
they all go equally long]
(6)
This comparative physiology was not confined to popular accounts. In Christiania (now known as Oslo), the Faculty of Medicine employed the same line of reasoning in 1847 when justifying to the Ministry the duration of a human pregnancy resulting in a live birth: 'likewise by analogy, drawn from reliable observations of domestic animals, particularly the gestation period of cows, which in days corresponds with that of humans […]' (13).
The obstetrician Kristian Brandt (1859–1932) continued to refer to the cow throughout the 20th century in his textbook for midwives (several editions 1913–44): 'The cow is closest to the woman with regard to the duration of pregnancy; it is on average 285 days, but ranges between 240 and 321 days' (14. 67).