BookmarkBookmarkTick

Q&A: How Do Twins Develop?

It’s twins! How did they develop?
BySandra Le Plasse
Updated
March 2, 2017
Hero Image

If you’re pregnant with twins, you’ve probably spent more than a few minutes wondering how planning for one baby suddenly turned into planning for two. Well, whether it’s a result of your age, genetics, or fertility treatments, having twins is something of a miracle (in fact, they make up a mere 3 percent of all pregnancies). Even if you plan to dress your babies in matching onesies (a sure way to elicit an “awwww!” from strangers), their similarities in appearance depend heavily on their development.
 
Identical twins develop after your fertilized egg splits in half and creates two genetically identical (aka monozygotic) embryos. If this happens a couple of weeks after conception, then each baby gets his or her own gestational sac and placenta. If they split up later, the two embryos will probably share the same sac (consider it a prebirth lesson in sharing!).
 
Fraternal twins (or nonidentical twins) develop when two separate sperm fertilize two separate eggs. This creates two genetically different (dizygotic) embryos. In this case, the only thing the babies will be sharing (for now) is your uterus (give them some credit — it’s not that big!).

American College of Obstetrics and Gynecologists. Your pregnancy and birth. 4th ed. Washington, DC: ACOG; 2005.

 

Related Video
mother interacting with her daughter at the hospital after conjoined twin surgery

Rare Set of Conjoined Twins Successfully Separated After 24-Hour Surgery

profile picture of Nehal Aggarwal
Nehal Aggarwal
Associate Editor
Published
11/02/2020

Q&A: Vanishing Twin Syndrome?

profile picture of Karen Moise, RN
Karen Moise, RN
Registered Nurse

Q&A: Can Twins Kick Each Other?

profile picture of Karen Moise, RN
Karen Moise, RN
Registered Nurse
newborn twins ermbracing

Oldest Woman to Give Birth Delivers Twin Girls at 73 Years Old

profile picture of Nathalie Kirby
Nathalie Kirby
Assistant Social Media Editor
twins in their crib

Woman Gives Birth to Twins—3 Months Apart

profile picture of Ashley Edwards Walker
Ashley Edwards Walker
Contributing Writer
Published
08/27/2019
red balloon letters that spell out love with collage of blue hearts

Preemie Twins Hug to Celebrate First Time Reunited Since Mother's Womb

profile picture of Stephanie Grassullo
Stephanie Grassullo
Associate Editor
Published
03/07/2019
several images of one woman's pregnancy progression over time

Mom Documents Triplet Baby Bump to Show Her Extraordinary Progression

profile picture of Stephanie Grassullo
Stephanie Grassullo
Associate Editor
Published
09/12/2018

Passing Your Due Date With Multiples

profile picture of Excerpted from The Baby Bump, Twins and Triplets Edition
Excerpted from The Baby Bump, Twins and Triplets Edition
closeup big pregnant belly

Preventing Premature Birth With Multiples

profile picture of Excerpted from The Baby Bump, Twins and Triplets Edition
Excerpted from The Baby Bump, Twins and Triplets Edition
Article removed.