Meaning two chorions(outer sac) and two amnions (inner sac) (two placentas, usually fused together, but not always)
Each baby has their own chorion, amnion and placenta
For identical twins the egg splits between day 0 and day 3
2. Mono/Di types of twins: identical
Meaning one chorion and two amnions (one placenta)
Each baby has their own amnion, but share a chorion and a placenta
The egg splits between day 4 and day 7
3. Mono/Mono types of twins: identical and conjoined
One chorion and one amnion (one placenta)
Both babies share both sacs and placenta
The egg splits between Day 7 and day 14, the later the split the higher chance of conjoined twins occurring
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.
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).
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.
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.
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.
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.
MONOZYGOTIC (IDENTICAL) TWINS
Identical: known as Monozygotic
A single fertilized egg splits into two embryos.
Because the twins share the same DNA set, they tend to have similar features.
They share the same placenta and are always of the same sex.
HOW DOES IT HAPPEN?
Occasionally a single fertilized egg forms a zygote that divides to form two separate embryos during the first 2 weeks after conception. These twins are referred to as monozygotic or "identical" twins. Depending on the timing of the division each zygote may develop its own membranes and placenta or the embryos may share the membrane and placenta. If the division occurs very late the twins will be conjoined.
PLACENTAL TYPES
1. Dichorionic (two placentas), Diamniotic (two amnions)
( 18 to 36 % of monozygotic twins )
If a single zygote divides into two separate embryos within 72 hours after fertilization then the twins
will develop in two separate amniotic sacs (bag of water) and will have their own, separate,
placentas. However, the placentas may at times appear to be fused.
2. Monochorionic (one placenta), Diamniotic (two amnions)
( 60 to 70 % of monozygotic twins )
If division occurs 3 to 8 days after fertilization then the twins will develop in two separate amniotic sacs (bag of water) and will share one placenta.
3. Monochorionic (one placenta),
Monoamniotic (one amnion)
( 1 to 2 % of monozygotic twins )
If division occurs 8 to 12 days after fertilization then the twins will share one amniotic sac (bag of water) and will share one placenta.
4. Monochorionic (one placenta),
Monoamniotic (one amnion)
( 0.5 % of monozygotic twins )
If division occurs 12 to 13 days after fertilization then the twins will share one amniotic sac (bag of water) and will share one placenta. They will also share body parts.
Conjoined twins occur 1 in 40,000 total births.
ULTRASOUND FINDINGS
It is not uncommon to "miss" a monochorionic twin on ultrasound examination and diagnose these pregnancies as a singleton early.
Ultrasound findings which support the diagnosis of monozygotic twins include:
Single placental site
Same-sex fetuses
Thin dividing membrane (two layers or <2 style="font-weight:bold;">FACTORS INFLUENCING THE OCCURRENCE OF MONOZYGOTIC TWINS
The birth rate of monozygotic twins worldwide appears to be relatively constant at 3 to 5 per 1000 births.
Infertility therapy
Monozygotic twinning has been reported to increase after ovulation induction techniques and in proportion to the number of blastocysts transferred during in vitro fertilisation.
Maternal or paternal history of monozygotic twinning
Monozygotic twinning transmitted either through the father or mother has been
reported.
Identical Twins....Hereditary?
Study: Identical Twins Caused by 'Embryo Collapse'
Thursday, July 05, 2007
LYON, France — Scientists proposed a new theory this week for how identical twins are formed as embryos, in a discovery that may improve a broad range of artificial reproduction techniques.
Using specialized computer software to take photos every two minutes of 33 embryos growing in a laboratory, Dr. Dianna Payne, a visiting research fellow at the Mio Fertility Clinic in Japan, and colleagues documented for the first time the early days of twin development.
The research was presented Monday at a meeting of the European Society for Human Reproduction and Embryology in Lyon, France.
• For more info, visit FOXNews.com's Human Body Center.
Payne and colleagues found that identical twins are formed after an embryo essentially collapses, splitting the progenitor cells — those that contain the body's fundamental genetic material — in half.
That leaves the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop.
"This is very exciting because it explains the principle of how we get twins," said Dr. Soren Ziebe, an artificial reproduction expert at the University of Copenhagen, who was not connected to Payne's research.
"Until now, we have assumed that at some point when the embryo breaks out of its shell to adhere to the uterus, it somehow gets divided into two," Ziebe said.
Payne's theory presents a plausible explanation of how that division occurs, according to Ziebe.
"This is an exciting paper because by watching the embryo growing continuously, it has given a new insight into the processes by which the embryo divides," said Dr. Alison Murdoch, a professor of reproductive medicine at Newcastle University's Fertility Centre.
Murdoch was not connected to Payne's research.
While conducting the research, Payne also found a possible explanation for why in-vitro fertilization techniques are more likely to create twins.
Only about three pairs of twins per thousand deliveries occur as a result of natural conception. But for IVF deliveries, there are nearly 21 pairs of twins for every thousand.
To date, scientists have been at a loss to explain the discrepancy.
Payne suggests that the laboratory conditions in which embryos are grown — in solutions that attempt to reproduce the uterus environment — are different enough to somehow provoke the development of twins.
"There are a lot of very subtle effects in the body, like different gas concentrations, enzymes or salt solutions," said Payne.
While scientists attempt to replicate an embryo's natural environment, they are still far off.
"The difference between the lab conditions and what's present in the body is probably resulting in more twins," she said.
"We need to take careful account of this new information," Murdoch said, adding that growing embryos for longer periods in the laboratory may be adding to the likelihood of having twins.
Payne suggested that with more research, doctors could potentially devise a test to predict which embryos might be inclined to produce identical twins.
Because carrying twins is more risky than carrying a single baby, experts think the success rates of artificial reproduction would increase if twins could be avoided.
In the photos from Payne's research, scientists were able to observe two distinct "inner cell masses" in the embryos that developed twins.
Of the 26 embryos that developed enough for them to observe, two had these distinct internal cell masses that scientists think are characteristic of twins.
Experts think that Payne's discovery could be applied to a number of artificial reproduction techniques.
"If we can improve the laboratory conditions for embryos to minimize the development of twins, then we can optimize the conditions for all eggs and embryos in artificial reproduction," said Ziebe. "This research has the potential to help all IVF procedures."
DIZYGOTIC (FRATERNAL) TWINS
Fraternal: known as Dizygotic
Two eggs fertilized by two different sperm.
Each has its own placenta, and they can be the same sex or not.
HOW DOES IT HAPPEN?
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.
Trizygotic triplets occur when three separate eggs are fertilized by three separate sperm.
Dr. Curran is a board-certified perinatologist with the San Gabriel Valley Perinatal Medical Group , Calif
PLACENTAL TYPES
Dichorionic (two placentas), Diamniotic (two amnions)
Dizygotic (fraternal) twins develop in two separate amniotic sacs (bag of water) and will have their own, separate, placentas. However, the placentas may at times appear to be fused.
ULTRASOUND FINDINGS
Ultrasound findings which support the diagnosis of dizygotic twins include:
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.
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.
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.
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.
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.
Multiparity
The risk of having dizygotic (fraternal) twins is increased in women with several previous pregnancies.
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.
3. Semi-Identical:
Two sperm cells fertilized one egg—an event assumed to be very rare—then splits into two embryos.
4. Polar Body Twins:
The egg splits before fertilization. Although a case of semi-identical twinning has been confirmed, polar body twinning
remains a theory.
Source: http://www.livescience.com/health/070326_semi_twins.html
Source: http://multiples.about.com/cs/glossary/g/polarbody_def.htmL
Source: http://www.obfocus.com/high-risk/twins/twin1.1.htm
Posts
