Detecting Twins with Ultrasound

As recently as a generation or two ago, twins were very often a surprise at the time of delivery. Fortunately, thanks to ultrasound, twins (or more) are usually detected well before the 5th month. In fact, it is very possible to know even earlier than 8 weeks if you have an early ultrasound.  Early detection of twins (or more) is important since it allows you to get adjusted to the idea and make the necessary preparations to double up on cribs, car seats, etc. More importantly, it allows your doctor or midwife to anticipate and hopefully prevent some of the complications that tend to occur more often with twins such as preterm labor and gestational diabetes. Twins and multiple pregnancies are considered high risk compared to singletons, and more frequent monitoring and visits to your care provider are usually the norm.


How early can you see twins?

It is possible to tell if there was more than one conception as early as 5 to 6 weeks, which is the same time as you can tell a single pregnancy. Twins are very obvious at all stages of the pregnancy. Despite stories to the contrary, with current technology it is not really possible for one twin to “hide” behind the other. Early on, using transvaginal ultrasound two sacs can easily be identified, even before you can detect heartbeats within them. The pictures below show how obvious one vs. two vs. three sacs are at only 6 weeks. In some, you can see an embryo and/or a yolk sac which is a little bubble of nutrients that can be seen by 5.5-6 weeks. The purpose of a yolk sac is to provide nutrients to the embryo while the placenta is forming. It then becomes obsolete by 12 weeks and eventually disappears.

Single sac with little 6.5 week embryo inside


Two gestational sacs, two yolk sacs = twins!

3 sacs = triplets!

The following image shows a twin gestation a couple of weeks later, around 8 weeks. An embryo can be seen within each sac. The development of an embryo with a heartbeat is a good hurdle, since most miscarriages occur because an embryo fails to develop to this point (seemiscarriages).



8 week twins, each with its own sac and thick dividing wall.



The earlier ultrasounds (up to 13 weeks) with twins are often the better ones as far as being able to get cute pictures of both babies together. As the babies grow, it becomes very hard to get good clear pictures of each one because there are twice the number of limbs and body parts to get in the way of a good view of the face.

2D image of 12 week twins







12 week 3D

Identical versus fraternal twins:

It is possible very early on to tell if the twins are identical versus fraternal. As described in the section on identical vs. fraternal twins, if the twins are sharing a placenta or an amniotic sac, they must be identical. If they both have their own sacs and placenta, and they are the same sex, you can’t be sure if they are identical until after birth. They PROBABLY are fraternal since that is a much more common type of twinning—- only 8% of twins are identical.

6 weeks, note there are two yolk sacs and one visible embryo tucked at the side. Two yolk sacs inside one gestational sac always means identical


2 weeks later, at 8 weeks we see two embryos sharing a single gestational sac. Each has it's own amniotic bubble around it, but sharing the single large sac always means identical twins.


  1. how can detect to male&female

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