A twin pregnancy most commonly occurs when two separate eggs are fertilized by separate sperm to form two zygotes. Each zygote implants in the uterus individually and develops its own membranes and placenta.  The two zygotes continue to develop as two separate embryos. These twins are referred to as dizygotic (commonly known as fraternal) twins. Fraternal twins account for about 70 % of naturally occurring  twins in the United States [1].

Trizygotic triplets occur when three separate eggs are fertilized by three separate sperm.

Twins may also occur  if a single fertilized egg forms a zygote that divides to form two separate embryos. These twins are referred to as monozygotic or "identical" twins. Identical twins account for about 30% of naturally occurring  twins in the United States [1]

Monozygotic (identical) triplets may occur if a single fertilized egg forms a zygote that divides to form three separate embryos .

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 monozygotic twins).

Dr. Curran is a board-certified perinatologist with the San Gabriel Valley Perinatal Medical Group , Calif

SUPERFECUNDATION

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

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 superfecundation.[4]

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 embryos [7]. 

Loss of one twin in the first trimester does not appear to impair the development of the surviving twin [8]. 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].

ULTRASOUND FINDINGS

Ultrasound findings which support the diagnosis of dizygotic twins include [11,12]: Separate placental sites Opposite-sex fetuses A "twin peak" sign Produced by fingerlike projections (villi) extending between the membranes from the placenta. Thick dividing membrane (four layers > 2mm)

FACTORS  INFLUENCING  THE  OCCURRENCE  OF  DIZYGOTIC  TWINS

• Maternal age

  The rate of twinning appears to increase with maternal age reaching a peak at about 37 years of age [13].
   

• Infertility therapy

  Over the last twenty years the number of twin and higher-order multiple births has increased to record numbers [14]. The rising incidence of multiple births has been associated with older age at child bearing and the increasing use of fertility therapies [15].
   

• Maternal history of twinning

It is believed that hereditary dizygotic twinning may be transmitted as a recessive trait [16] or as an autosomal dominant female-specific trait with limited expression [17] .

"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 " [18].

High follicular follicle-stimulating hormone level may explain the hereditary tendency for multiple ovulation and pregnancy in some families [19,20].
 

• Ethnicity

  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 [1].

  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 [21].
   

• Multiparity

  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 [24].