Multiple pregnancies and the Vanishing Twin Phenomenon
Medical research indicates that at least one in eight to ten births starts out as twins (Landy & Keith, 1998; Wright, 1997; Austermann, 2006), though only one in eighty to ninety will result in a liveborn twin pair (Wright, 1997). In assisted reproductive pregnancies, multiples fare much higher, sometimes up to 100%, depending on the method used (Bergh et al., 2000).
With common routine use of ultrasound at around week nine to twelve (with week nine the fetal stage begins), twin or multiple conceptions often go undiscovered due to a phenomenon that has been called the Vanishing Twin Syndrome (VTS). In 1945 Stoeckel "…was the first to suggest that the conception rate of multiple gestations was greater than the birth rate:
'It thus appears that twins are more often conceived than born; not only in addition to the evidence of foeti papyracei, it may be that twin material is reabsorbed due to early death, without leaving any trace.'" (Landy & Keith, 1998, p.177)
It was not until the 1980's that this topic received heightened interest, likely because of the increase in assisted reproductive procedures such as in vitro fertilization (IVF) and connected to it the interest in possible medical outcomes and risks, as well as the steady developments in sonographic technologies. (Landy & Keith, 1998) According to Wright (1997) "[t]wins are far more susceptible to birth defects, spontaneous mutations, and vascular problems that threaten early life. Simply being a twin is stressful and raises the odds against survival because of the competition for space and nutrients." (p.89) Twins or multiples are also at a higher risk than singletons to pre-term delivery, low birth-weight, and death in the neo-natal period. (Wright, 1997; Bryan, 2007)
There are an increasing amount of reports which tell us about the reality of the VTS:
While two or more fetuses were seen in early ultrasound, a later scan often reveals only one of the fetuses and/or an empty gestational sac. This first examination is generally not done before week five which is the first time when most transvaginal ultrasounds can pick up a gestational sac. Between the weeks 5 ½ to 6 ½ a fetal pole (the first visible sign of a developing embryo) or even heart may be detectable.
'Vanishings' typically occur within of the first nine gestational weeks, with later losses resulting in still birth or some fetal material being produced when the co-twin is born.
One reason for early loss may be that the embryo is far more sensitive to damage from environmental exposures than the fetus. Although cited as the most common cause of first trimester bleeding and/or cramping, many mothers will often not consciously detect any 'disturbance'. (Austermann, 2006; Steinemann, 2006)
The rare foetus papyraceus (or leaf-like fetus) develops when no reabsorption happens after the 20th gestational week.
The fetus will stay in the womb, drying out and in the process is flattened by the survivor. Delivered with the placenta at birth, it has been known to midwifes since centuries. Women in the Western world are seldom told when a foetus papyraceus or other fetal material turns up during delivery, and, as of this, may never know that they were carrying more than one. Even in cases when the mother was informed about the loss of one of her babies(), various reasons such as unprocessed grief, lack of societal acknowledgement, or simply ignorance may lead to the decision of not telling the survivor about the initial existence of a sibling in utero. (Austermann, 2006; Briscoe & Street, 2003)
Although once considered extraordinary, recent numbers suggest that we are dealing with a relatively common phenomenon which assures that at least one baby will be born; as of this we can hardly talk about a syndrome. (Bergh et al., 2000; Brokelage, 2007) The Australian therapist John James (2007) from The Crucible Centre even estimates that VT's may be as high as 25 percent (one in four) in people seeking out therapy. Not included in all mentioned estimates are the disintegration of 'multiples' before nidation in utero. This implantation process of the so called blastocyst(s) happens anywhere from day six to ten since conception; only 15 to 40 percent will even make it until there. (Austermann, 2006)
Forms of twinning
How and why twins occur is still a debate in science. What we do know is that there are in general two types of twins: dizygotic (or fraternal) and monozygotic (or genetically identical) twins. More than two thirds of twins are fraternal. They develop from two separate fertilized eggs, have their own placentas, chorion and amnion, and may even be fathered - sometimes by different fathers - at different times. (Austermann, 2006; Steinemann, 2006; Wright, 1997) Another rare variation are half identical (or polar body) twins. This "…occurs when one egg splits in two before fertilization. Each half is then fertilized by a separate sperm. These twins share 75% of their DNA as they share the same DNA from one egg and different DNA from each sperm." (Pregnancy-Info.net, 2007, n.p.)
Identical twins develop from one egg and one sperm which splits during the first two weeks after conception. Depending on the time of splitting, four possibilities exist:
- About 30 percent split within of the first three days after fertilization and will produce separate chorions and amnions. These dichorionic twins have different placentas that can be separate or fused.
- Later splitting between days four to seven accounts for 70 to 75 percent of the twins and results in a shared placenta and chorion, but two amnionic sacs.
- The rarest of identical twins (only one to three percent) will develop if splitting occurs between days eight to twelve. This means that they are sharing one amnionic sac and placenta with vascular communication between the two circulation systems. These twins are at high-risk to birth defects and twin-to-twin transfusion syndrome (TTTS).
- If twinning occurs on day 13, the pair only partially splits which will result in conjoined twins. (Austermann, 2006; Steinemann, 2006; Wright, 1997)