The proposition that a pregnancy is 40 weeks or 280 days in duration is attributed to the German obstetrician Franz Naegele (1778-1851). His rule adds nine months and seven days to the first day of the last menstrual period. The expected date of confinement from this formula is approximately right in the majority of cases. However, the idea that this rule can apply to every pregnant female - young or old, nulliparous or multigravida, Caucasian, Asian, African, or Indigenous - stretches credulity. In addition, many women regard the 40-week date as a deadline, which if crossed, may then place the baby under stress. Forty weeks is such a simple, round, convenient figure that it has proved difficult to challenge, despite criticism. Nonetheless, what might have been an appropriate formula in Germany in the 19th century deserves to be revisited in the 21st.
To review the length of pregnancy, in the light of current technology, in particular ultrasound scanning, and assisted reproductive techniques.
A Medline search was performed for variables on the length of pregnancy, the expected date of confinement, and prolonged pregnancy.
A number of factors were found to significantly influence the length of a pregnancy, including ethnicity, height, variations in the menstrual cycle, the timing of ovulation, parity and maternal weight.
Naegele\'s rule should be considered as a guideline for the expected date of confinement, and not a definite date.

Reproduction traits, such as gestation length (GLE), play an important role in dam line breeding in pigs. The objective of our study was to identify single nucleotide polymorphisms (SNPs) that are associated with GLE in two pig populations. Genotypes and deregressed breeding values were available for 2081 Dutch Landrace-based (DL) and 2301 Large White-based (LW) pigs. We identified two QTL regions for GLE, one in each population. For DL, three associated SNPs were detected in one QTL region spanning 0.52 Mbp on Sus scrofa chromosome (SSC) 2. For LW, four associated SNPs were detected in one region of 0.14 Mbp on SSC5. The region on SSC2 contains the heparin-binding EGF-like growth factor (HBEGF) gene, which promotes embryo implantation and has been described to be involved in embryo survival throughout gestation. The associated SNP can be used for marker-assisted selection in the studied populations, and further studies of the HBEGF gene are warranted to investigate its role in GLE.