Dewhurst's Textbook of Obstetrics and Gynaecology by Keith Edmonds

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However, dating by menstrual history has several problems [2]. First the last menstrual period (LMP) may not be accurately remembered in a substantial proportion of cases. Second, dating by LMP assumes that conception occurred in mid-cycle, whereas it may have occurred earlier or (more likely) later. If the usual length of a woman’s cycle tended to be longer, say 35 instead of 28 days, then an adjustment needs to be made by adding 7 days to the EDD. This is often not done, but even if it is, it represents an imprecise science as the actual length of the follicular phase at the beginning of that pregnancy is not known.

Ultrasound detection of placental insufficiency in women with ‘unexplained’ abnormal maternal serum screening results. Clin Genet 2006; 69: 97–104. 44. Chaddha V, Whittle WM & Kingdom JCP (2004) Improving the diagnosis and management of fetal growth restriction: the rationale for a placenta clinic. Fetal Matern Med Rev 15, 205–30. 45. Konje JC, Huppertz B, Bell SC, Taylor DJ & Kaufmann P (2003) 3-dimensional colour power angiography for staging human placental development. Lancet 362, 1199–201. Further reading For further reading on structural characteristics of the placenta, see [1] or [2]; the definition of fibrinoid, see [18]; trophoblast and its changes during pre-eclampsia, see [23]; a detailed descriptions on pathologies of the macroscopic features of the placenta, see [1]; the classification of villi and the types of villi, see [1]; stereological parameters of the growing placenta, see [20]; syncytial fusion and the involvement of apoptosis, see [22] and [23]; the impact of oxygen on placental development and placental-related disorders of pregnancy, see [16]; the composition and characteristics of fetal membranes, see [35]; rupture of fetal membranes, see [34]; placental assessment by ultrasound, see [37]; placental Doppler, see [40,43,44]; developmental placental pathology, see [1,37].

Length of pregnancy In any large database, the distribution of the length of pregnancy is skewed because babies are more likely to be born preterm than post-term and at a wider range of gestations into the early preterm period. Thus neither the mean nor median, but the modal value is used to denote the typical length of pregnancy. Starting from the time of conception, this typical length of gestation and the fetal age at the end of pregnancy is 266 days or 38 weeks (= conceptual age). In most (but by no means all) cases conception occurs in midcycle and thus 2 weeks are added to denote menstrual age.