Published online by Cambridge University Press: 23 February 2010
TYPES OF TWINS AND THEIR ORIGINS
For naturally conceived multiple pregnancy, about 70% are dizygotic (DZ) twins, resulting from double or multiple ovulations in the same cycle; this is attributed to higher levels of maternal follicle-stimulating hormone (FSH) or increased FSH receptor sensitivity. Monozygotic (MZ) twinning involves postzygotic splitting, resulting in two or more embryos. This can occur at any time up to about 14 days postconception.
A third type of twin, the “polar body” twin, is very rare.
DETERMINATION OF ZYGOSITY
Zygosity determination is best carried out by analysis of restriction fragment length polymorphisms in variable number tandem repeat sequences of DNA, extracted from chorionic villus samples, amniotic fluid cells, peripheral blood white cells, placenta, cord and membranes, and tissues sampled at autopsy, including macerated stillborn fetuses (Table 23.1).
PLACENTAL ANATOMY OF ZYGOSITY
Of MZ twins, about one-third have dichorionic (DC) placentas (separate or fused disks), while two-thirds have truly single monochorionic (MC) placentas (Figure 23.1). Most MC placentas have two amniotic cavities (MC, DA), while a minority (about 1% of all twins) have a single amniotic cavity (MC, MA). Conjoined twins are MZ, and most are MC, MA.
To save this book to your Kindle, first ensure [email protected] is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Find out more about the Kindle Personal Document Service.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.