I’m pretty sure twinning can occur even a little beyond the morula phase. In fact, I think it’s possible to tell roughly when the split occurred, because if it happened at or shortly after the zygote phase, one can see two fused chorions and placentas. If twinning took place later (say at the morula stage and beyond), there is a single chorion with no fusion, as well as a discrete placenta shared by both embryos/fetuses.
As I mentioned above, I’m pretty sure if twinning takes place late into the blastocyst phase, there’s high danger for them to be conjoined.
MandaJo
August 17, 2005, 12:19am
22
Loopydude:
I’m pretty sure twinning can occur even a little beyond the morula phase. In fact, I think it’s possible to tell roughly when the split occurred, because if it happened at or shortly after the zygote phase, one can see two fused chorions and placentas. If twinning took place later (say at the morula stage and beyond), there is a single chorion with no fusion, as well as a discrete placenta shared by both embryos/fetuses.
As I mentioned above, I’m pretty sure if twinning takes place late into the blastocyst phase, there’s high danger for them to be conjoined.
If they share a placenta, doesn’t that mean that they effectively ARE conjoned, since they share an organ?
ouryL
August 17, 2005, 7:34am
24
Loopydude:
I’m guessing they started as identical quads, and one of the quads spontaneously aborted early on. I really can’t think of another mechanism by which you would get multiples in an odd number. “Vanishing twin” is a pretty common phenomenon in multiple pregnancies (more common than bringing all multiples to term, in fact), so I imagine it’s not much of a stretch that there would be higher-order multiples that might “vanish”. (The embryos are said to “vanish” because they’re visible and viable during earlier ultrasounds, but appear small, misshapen, or even absent during later ultrasounds, and eventually even the gestational sac disappears).
Risk is most high for loss during the first trimester, usually due to some major chromosomal aberration, most typically a trisomy that is incompatible with life. In the case of multiples, as there are other embryos and their gestational sac(s) pumping out hormones to signal continued pregnancy, the symptoms of miscarriage can be much less severe than if a singleton is lost, or all multiples are lost simultaneously. Minor cramping and a just little bleeding or spotting are all that might indicate the loss of one or more of the embryos, and whatever is left is resorbed fairly quickly, making it difficult to know if another embryo even was there if it’s not caught on an earlier ultrasound. But, like I said, I can’t think of how one would get an odd number of identicals except via a “vanishing” embryo, so I’m guessing that’s the history.
They all would share a gestational sac that should be very evident at 12 weeks, and the individual embryos, with their yolk sacs, are also clearly visible by this time. One can easily see gestational sacs at least as far back as five weeks, and it might even be possible to see a yolk sac by then. Not long after this, the embryo is around the size of a grain of rice, and can be seen reasonably easily by ultrasound.
Crown-rump length (CRL) and pulse rate are among the best diagnostics for survival during the first trimester. Small CRL and embryonic bradycardia tend to be highly predictive of eventual loss of the embryo, especially if the numbers are well below normal, and spontaneous abortion usually happens before week 16. Again, such abnormalities frequently correlate with aneuploidies (esp. trisomies), and the damage was likely done somewhere during the first few divisions. If some of the multiples are looking small, the prognosis for their survival is indeed poorer than if they were within a normal range.
Some vanishing twins are actually absorbed by the surviving twin! :eek: :eek: :eek:
There have even been identical quints. The Dionnes of Canada were identical, but the Diligentis of Argentina were fraternal. I’m sure that many, many more fraternal multiple births are being seen now due to fertility treatments.