Anemia of Prematurity - Portal Neonatal

occurs even later (ie, during days 9-12 after fertilization), then monochorionic/monoamniotic
placentation occurs (see Image 3). Monochorionic/monoamniotic twins are rare; only 1% of
monozygotic twins have this form of placentation. Monochorionic/monoamniotic twins have a
common placenta with vascular communications between the 2 circulations. These twins can
develop twin-to-twin transfusion syndrome (TTTS). If twinning occurs beyond 12 days after
fertilization, then the monozygotic pair only partially split, resulting in conjoined twins.
Triplets can be monozygotic, dizygotic, or trizygotic. Trizygotic triplets occur when 3 sperm fertilize 3
ova. Dizygotic triplets develop from one set of monozygotic cotriplets and a third cotriplet derived
from a different zygote. Finally, 2 consecutive zygotic splittings with 1 split result in a vanished fetus
and monozygotic triplets.
It is important to evaluate the placenta(s) after the birth of all multifetal pregnancies in order to
determine zygosity.
Frequency:
• In the US: The birth rate of monozygotic twins is constant worldwide (approximately 4 per
1000 births). In contrast, dizygotic twinning is associated with multiple ovulation, and its
frequency varies among races within countries and is affected by maternal age (increases
from 3 in 1000 in women younger than 20 years to 14 in 1000 in women aged 35-40 years,
declining thereafter) and parity. In the United States, the overall prevalence of twins is
approximately 12 per 1000, and two thirds are dizygotic. The birth rate of dizygotic twinning
is highest for African Americans (10-40 per 1000 births), followed by Caucasians (7-10 per
1000 births) and Asian Americans (3 per 1000 births). The rate of higher order multiple births
has also increased recently, which has been attributed to in vitro fertilization and embryo
transfer. Naturally occurring triplet births occur in approximately 1 per 7000-10,000 births;
naturally occurring quadruplet births occur in 1 per 600,000 births.
• Internationally: The birth rate of monozygotic twins is constant worldwide (approximately 4
per 1000 births). Birth rates of dizygotic twins vary by race. The highest birth rate of dizygotic
twinning occurs in African nations, and the lowest birth rate of dizygotic twinning occurs in
Asia. The Yorubas of Western Nigeria have a frequency of 45 twins per 1000 live births, and
approximately 90 percent are dizygotic.
Mortality/Morbidity: Multifetal pregnancies are high-risk pregnancies. The fetal mortality rate for
twins is 4 times the fetal mortality rate for single births. The neonatal mortality rate for twins is 6
times more than the neonatal mortality rate for single births. Higher order multiple births have even
greater mortality rates in comparison to twin and single births.
A high prevalence of low-birth weight infants, due to prematurity and intrauterine growth retardation
(IUGR) and their associated complications, contribute to this problem. Twins have increased
frequency of congenital anomalies, placenta previa, abruptio placenta, preeclampsia, cord accidents,
and malpresentations, as well as asphyxia/perinatal depression, group B streptococcal (GBS)
infections, hyaline membrane disease, and TTTS.
Race: The frequency of naturally occurring twin births varies by race. Black women have the highest
birth rate of twins, followed by Caucasian and Hispanic women. Asian women have the lowest birth
rate of twins. There is a racial disparity in the United States between black and Caucasian twin
stillbirths. Risk of stillbirth is elevated in black fetuses compared with white fetuses among twins but
not triplets.
Age: Maternal age has no effect on monozygotic twin births. Advanced maternal age (>35 y) is
associated with increased risk of dizygotic twins. Prevalence of naturally occurring twin births has
increased recently because of the trend to delay childbearing to later years.