Any combination of the above processes may occur in the creation of higher
order pregnancies (triplets, quadruplets, etc.) . For example dizygotic triplets
may develop from two separate eggs fertilized by separate sperm to form two
zygotes. One of the zygotes then divides to form two separate embryos (as for
Dr. Curran is a
board-certified perinatologist with the San Gabriel Valley Perinatal Medical
Group , Calif
Superfecundation refers to fertilization of two or
more ova (eggs) during the SAME menstrual cycle by sperm from separate
acts of sexual intercourse. Twins with different fathers are thought to arise by
this mechanism [2,3].
Superfetation is the fertilization of two or
more ova (eggs) during DIFFERENT menstrual cycles. This process is not
thought to occur naturally in humans, but might occur using assisted
reproductive technologies (ART). Superfetation is sometimes confused with
THE "VANISHING" TWIN
There are more twins
conceived than are born. Loss of a twin during the first
trimester appears to be a relatively common occurrence [5-7] and is
known as vanishing twin syndrome. The loss of one
twin can be expected in about 40% of pregnancies identified with twin sacs or
Loss of one twin in the first trimester does not appear to impair the
development of the surviving twin . However, a deceased co-twin may cause an elevated amniotic fluid alpha-fetoprotein AFP and
acetylcholinesterase (AChE) test result in an apparently normal surviving
fetus [9, 10].
DIZYGOTIC (FRATERNAL) TWINS
Dichorionic (two placentas), Diamniotic (two amnions)
Dizygotic (fraternal) twins develop in two separate amniotic sacs (bag of
will have their own, separate, placentas. However, the placentas may at
times appear to be fused.
FACTORS INFLUENCING THE OCCURRENCE OF DIZYGOTIC
- Maternal age
The rate of twinning appears to increase with maternal age reaching a peak at
about 37 years of age .
Over the last twenty years the number of twin and
higher-order multiple births has increased
to record numbers . The rising incidence of multiple births has
been associated with older age at child bearing and the increasing use of fertility therapies
Maternal history of twinning
It is believed that hereditary dizygotic twinning may be transmitted as a
recessive trait  or as an autosomal dominant female-specific trait with
limited expression  .
"The probability of a subsequent twin pregnancy is increased 4-fold in
mothers of twins, and the risk of having dizygotic twins is roughly doubled
for women whose mother or sister has dizygotic twins " .
High follicular follicle-stimulating hormone level may explain the hereditary
tendency for multiple ovulation and pregnancy in some families [19,20].
Dizygotic twinning rates vary by ethnic group and geographic area. In Nigeria
dizygotic twinning occurs at a rate of 49 twins per 1000 births. In Japan
the rate of dizygotic twinning is 1.3 per 1000 births .
In general twinning rates appear to also be modified by both migration and
interethnic mixing. A comparison of twinning rates of 14 ethnic groups in
California found twinning rates per 1000 maternities of 13.20 for blacks, 10.05
for whites, and 7.18 for Asians. Rates for Indians from India were lower
than those reported from the Indian subcontinent .
The risk of having dizygotic (fraternal) twins is increased in women with several
previous pregnancies [22,23].
High body mass index and height.
The odds of dizygous twinning is greater among women very tall women and in
women with a prepregnant body mass index (BMI) of 30 or more .
Monozygotic (Identical) Twins: How does it happen?
Twins- Preterm Labor
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