Which Twin Was Conceived First? Twin Conception Explained

| Reviewed By Amanda Lundberg, BSN, RN

If one finds out they are pregnant with twins, it is normal to have numerous inquiries. Some may be easy to answer while others may be more difficult to address. There could even be certain questions that cannot be resolved.

How do you know which twin was conceived first? In most cases of twins, you won’t know who was conceived first. Identical and fraternal twins are not conceived the same way, but there’s no easy way to know who was first in either situation. Unless the circumstances are very unique, you will just have to wonder.

Fortunately, you can learn so much more about your twins today than you could years ago. 

Twins and Conception – What To Know

How your twins were conceived will depend on what type you are having. 

How Identical Twins Are Conceived

Identical twins are conceived when one sperm fertilizes one egg. The egg then splits, and now you have identical twins. Since the egg is fertilized before it splits, neither twin is technically conceived first.

How Fraternal Twins Are Conceived

Fraternal twins occur when two eggs are released instead of one. Two separate sperm then fertilize each egg.

Technically, one egg could be fertilized hours or days before the next one is, but the time between will probably be too small for there to be a major difference in your children. That’s why it can be hard to know who was conceived first.

What Does It Mean if Twins Are in Different Sacs?

If twins don’t share a chorionic or amniotic sac, they are fraternal twins. Since fraternal twins come from two different eggs, they don’t share these sacs and instead have their own space and placenta.

Identical twins can be a bit different. Mo-di twins have their own amniotic sac, but they do share a chorionic sac. That means they both draw nutrients from the same placenta.

Mo-mo twins, another type of identical twins, share both an amniotic and chorionic sac. This type of pregnancy is rare and very high risk.

Can Twins Be Conceived on Different Days?

Identical twins will be conceived on the same day, but fraternal twins can be conceived on different days. In fact, fraternal twins can even have different fathers.

Twin Position in Uterus

Twins can assume a variety of positions in utero, and they will also move around a lot throughout the pregnancy.

The most common positions are: 

  • vertex/vertex (both head down)
  • vertex/breech (one head down, one head up)
  • transverse (both babies lying sideways across mom’s belly)

The position your twins are in as birth nears will help your doctor decide the best birth plan for everyone’s safety.

A woman is being given an ultrasound to check on her pregnancy.

How Doctors Determine Baby A and Baby B

The twin assigned the title of Baby A is closest to the cervix. The logic is that Baby A will come out first during a vaginal delivery, and that’s why he gets the first letter of the alphabet. Baby B is the one who is not as close to the cervix.

While this can be a good way to identify twins in the womb, they can also change positions.

In my case, Baby A was expected to come out first until the doctor randomly grabbed Baby B during my cesarean section, helping her escape the womb before her sister.

Which Twin Comes Out First?

The twin closest to the cervix will likely come out first during labor. While babies can still turn to breech or transverse positions during labor, it’s hard for one to move closer to the cervix when labor has already started and Baby A has dropped.

During a cesarean section, it’s anyone’s guess. If your doctor believes one baby will need immediate medical care, that may be the one who gets taken first.

Related Questions: 

How Rare Are Fraternal Twins?

Fraternal twins aren’t as rare as identical twins. Almost 25 of every 1,000 births are fraternal twins. 

Do Twins Have the Same Personality?

Take it from someone who has identical twins: twins do not have the same personality.

However, some studies have shown that twins who were separated at birth were startlingly similar as adults. Still, fraternal and identical twins are their own people, and they won’t act exactly the same.

Final Thoughts

Twins are fascinating, and there is still so much we don’t know about them, including who is conceived first.