單絨毛膜雙羊膜類的雙胞胎，則會出現雙胎輸血症的風險，其中一個胎兒會搶去另一胎兒的血液。

雙胞胎可分為兩大類：異卵雙胞和同卵雙胞。

異卵雙胞胎 Fraternal twins

顧名思義，異卵雙胞是由兩條精子分別與兩粒卵子結合而來，兩個胚胎各自帶著不同的遺傳基因，兩個胎兒的性別也不一定相同；一男一女的「龍鳳胎」就是異卵雙胞胎了。

通常被人们称为假双胞胎”。

即使是相同性別，異卵雙胞的「孖仔/孖女」外貌也不會完全一樣。

異卵雙胞的胎盤屬於雙絨毛膜雙羊膜類，每個胎兒均有獨立的絨毛膜和羊膜；意思是在媽媽的子宮內，兩個胎兒各自有「獨立的房間」，所以他們互相干預的機會很低。

同卵雙胞胎 Identical twins

同卵雙胞則是由一條精子與一粒卵子結合而來，受精後的胚胎一分為二，分裂出來的兩個胚胎有相同的遺傳基因，兩個胎兒的性別也相同，一定不是「龍鳳胎」。

即他们具有相同的基因，所以出生后的双胞胎性别相同、容貌相似，血型、体质等也相同，这通常被人们称为真双胞胎”。

同卵雙胞的「孖仔/孖女」外貌會完全一樣。同卵雙胞的胎盤類型又可細分為雙絨毛膜雙羊膜、單絨毛膜雙羊膜或單絨毛膜單羊膜三種。

單絨毛膜單羊膜類是最危險的，因為這類雙胞胎的兩個胎兒沒有羊膜分隔，在媽媽的子宮內沒有「獨立房間」，所以他們「打架」或互相干預的機會較大。他們的臍帶可能會纏結，導致缺氧。最極端的狀況就是聯體孖胎，俗稱「連體嬰」。

18%的同卵雙胞胎有不同的優勢手：比如哥哥擅長左手，弟弟可能擅長右手。

雙胞胎會有相同的指紋嗎？ 其實對於雙胞胎來說，他們可以有一樣的臉、一樣的生日、性格、甚至DNA，但是相同的指紋是少之又少，但是生命總會有奇蹟，說不定世界上某個角落會有相同指紋的雙胞胎，只是這種概率非常之小。

雙胞胎會有自己的語言嗎？ 答案是會的。 科學表明，雙胞胎之間會有自己獨特的語言和交流方式，並且父母是無法破譯的。

幾乎一半的雙胞胎都有自己的語言：為了相互交流，大約40%的雙胞胎創造了一種只有他們自己理解的語言。

雙胞胎會有不同的生日嗎？ 會有不同生日的雙胞胎。 比如第一個寶寶在23：59出生，而另一個在第二天的0:00出生，這就擁有不同的生日啦！

雙胞胎有心電感應嗎? 答案是有的!

mental telepathy 心電感應 心電感應的天賦是否存在這些人的潛意識中，和所傳達的訊息可以被儲存並且由光子傳輸給對方．在雙胞胎的實驗當中，許多系列的測試已非常清楚證明此類的兄弟可以相互溝通，而溝通的方式卻無法以正統物理學去加以解釋的，雙胞胎有著完全相同的遺傳因子，很明顯地會大大增加心電感應的技巧。

實事上，縱然感受不是很強烈但我們每天的日常生活亦會有心電感應的感覺

一對雙胞胎被分離在不同的孤兒院彼此互不認識，卻試圖著和對方聯繫而終於找到對方，特別是在雙胞胎其中一位遭遇到危急狀態時，心電感應的潛在力量非常戲劇性的被發揮出來．

個人例子: 在我幼稚園的時候,有一次我開開心心和其他小朋友玩玩具莫名其妙突然大哭,老師也百思不得其解...後來老師問我我只說我要找妹妹,老師帶我到妹妹的班級,發現妹妹一直哭。 我哭因為妹妹在哭。

爸媽不同種族的話，雙胞胎看起來像是不同種族的機會只有百萬分之一

夫妻 Remi Horder 和 Kylee Hodgson 真的就有一對這樣的雙胞胎寶寶。Kian 是黑皮膚、深咖啡色頭髮、深咖啡色眼珠，但是 Remee 是白皮膚、金頭髮、藍眼珠。

從綜合研究結果看起來，同卵雙胞胎一起出櫃的機會大約在五成左右，而異卵雙胞胎性傾向的相關性就比較低，降到兩成左右；也就是說，當異卵雙胞胎的其中一個出櫃時，另一位也出櫃的機會大約20%左右。

基因愈相同，性傾向愈一致

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