**5.7. Stillbirth**

Advanced maternal age has been associated with an increased incidence of stillbirth [91]. A mechanism under this increase is placental dysfunction, which accounts for around 65% of stillbirths, and it has been observed more frequently in mice models and humans with age. Placentas from older mothers (35–39 and ≥ 40 years old) are less efficient in the sense that foetal/placenta weight ratio was lower than placentas from controls under 30 years old. They seem to be bigger in size and display mechanisms to ameliorate function, like increased relaxation of myometrium arteries and increased amino acid transport, but this does not correlate with a higher birth weight in the offspring. The hypothesis is that an increased size could be an adaptive mechanism trying to make up for placental dysfunction [92]. It has also been suggested that the greater contribution to stillbirth in older mothers could arise from their increased risk of chromosomal abnormalities [80].

Twin pregnancies are also high risk for stillbirth and neonatal death, increasing thirteenfold in monochorionic and fivefold in dichorionic pregnancies compared to singletons [93, 94].

Although this is not under the scope of this chapter, advanced paternal age has also been associated with stillbirth and death of the child before 5 years of age [95, 96]. The risk might be linked to a higher rate of sperm chromatin or chromosomal aberrations. Interestingly, this association dissolves when adjusting for paternal education level, when the association between advanced maternal age and the risk of stillbirth is independent of socioeconomic and educational levels.
