1. Refers to maternal age at expected time of birth. In the case of an IVF pregnancy, maternal age is guided by the age at egg retrieval (whether own egg or donor egg)

2. Dizygotic twins: Both twins originate from a separate oocyte and each oocyte is fertilized by its own spermatocyte. All dizygotic twins are dichorionic/diamniotic.

3. Monozygotic twins: Both twins develop from a single zygote which then cleaves to form two embroys. Majority of monozygotic twins are monochorionic/diamniotic. Depending on the timing of the cleavage, monozygotic twins may be dichorionic/diamniotic or monochorionic/monoamniotic.

*Refers to maternal age at expected time of birth. In the case of an IVF pregnancy, maternal age is guided by the age at egg retrieval (whether own egg or donor egg).

Tables Generated Based on Data From:

1. Morris JK, Wald NJ. Estimating the risk of Down's syndrome in antenatal screening and the gestation at which this risk applies. J Med Screen. 2007;14(1):5-7. doi: 10.1258/096914107780154549. PMID: 17362564.

2. Boyle B, Morris JK, McConkey R, Garne E, Loane M, Addor MC, Gatt M, Haeusler M, Latos-Bielenska A, Lelong N, McDonnell R, Mullaney C, O'Mahony M, Dolk H. Prevalence and risk of Down syndrome in monozygotic and dizygotic multiple pregnancies in Europe: implications for prenatal screening. BJOG. 2014 Jun;121(7):809-19; discussion 820. doi: 10.1111/1471-0528.12574. Epub 2014 Feb 4. PMID: 24495335; PMCID: PMC4234000.