# Twins **URL:** \ **Category:** Factual Questions\ **Created:** [December 17, 2010, 2:38am UTC](https://boards.straightdope.com/t/twins/564462 "2010-12-17T02:38:09Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Foggy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/foggy/32/1082_2.png) [@Foggy](https://boards.straightdope.com/u/Foggy)\ **Post date:** [December 17, 2010, 2:38am UTC](https://boards.straightdope.com/t/twins/564462/1 "2010-12-17T02:38:09Z")
The author/artist of a webcomic I follow, announce today that: > [@](#): > > My wife is pregnant with identical twins, and while the pregnancy was not a surprise, the number of pregnancies was. I am equal parts thrilled and terrified. They’re due in June or thereabouts, so I have some time to get my head on straight before fetal armageddon arrives. How can they know that they are identical at this stage? ---
**Author:** ![Shmendrik](https://avatars.discourse-cdn.com/v4/letter/s/ee7513/32.png) [@Shmendrik](https://boards.straightdope.com/u/Shmendrik)\ **Post date:** [December 17, 2010, 2:50am UTC](https://boards.straightdope.com/t/twins/564462/2 "2010-12-17T02:50:38Z")
> [@Foggy](#): > > The author/artist of a webcomic I follow, announce today that: > > How can they know that they are identical at this stage? Maybe ultrasound shows that they share a single placenta or amniotic sac. ---
**Author:** ![delphica](https://avatars.discourse-cdn.com/v4/letter/d/e9a140/32.png) [@delphica](https://boards.straightdope.com/u/delphica)\ **Post date:** [December 17, 2010, 3:08am UTC](https://boards.straightdope.com/t/twins/564462/3 "2010-12-17T03:08:18Z")
There’s always the possibility he’s assuming they’re identical for no particular reason, or perhaps identical twins run in the family so they feel safe making this assumption. Another thing might be if the couple had elected to have CVS or amnio testing done. That produces a very detailed genetic profile. Usually it’s enough to determine if the twins are identical or fraternal, although in some cases there isn’t enough material collected to make a determination. Some people get these tests done because they want as much information as possible, others are specifically worried about a particular genetic condition – so that can be stressful and they may not feel like sharing that part, even if the results were good. Also, if they are clearly sharing a placenta (which would be observable by this point), they are identical. If there are two placentas, they could be either. This would be something you would see in an ultrasound. This is subject to how good the technician is at interpreting the ultrasound images, so human error can be a factor. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [December 17, 2010, 3:47am UTC](https://boards.straightdope.com/t/twins/564462/4 "2010-12-17T03:47:30Z")
> [@Shmendrik](#): > > Maybe ultrasound shows that they share a single placenta or amniotic sac. Wikipedia sez: “In the uterus, a majority of identical twins (60–70%) share the same placenta but have separate amniotic sacs. In 18–30% of identical twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of identical twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.” So for some combinations of placenta vs. sac, it makes it more or less _likely_ that they are identical, but not a certainty. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [December 17, 2010, 5:24am UTC](https://boards.straightdope.com/t/twins/564462/5 "2010-12-17T05:24:35Z")
> [@thelurkinghorror](#): > > So for some combinations of placenta vs. sac, it makes it more or less _likely_ that they are identical, but not a certainty. If they share the same placenta, it is virtually certain they are identical. If they have separate placentas, they could be either identical or fraternal. ---
**Author:** ![Foggy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/foggy/32/1082_2.png) [@Foggy](https://boards.straightdope.com/u/Foggy)\ **Post date:** [December 17, 2010, 8:28am UTC](https://boards.straightdope.com/t/twins/564462/6 "2010-12-17T08:28:50Z")
Thanks everyone! I didn’t know such details would be know so early, but then I decided at the age of 10 to never have kids of my own and have never given it any further thought.