Nearly every patient I meet wants twins.  Of course, they want twins!  The idea of having two children at once when they have longed for children for years, makes perfect sense.  Many people want a sibling for the children, so instant siblings seems very efficient.


Twins happen for a few reasons:


  • spontaneously when a woman releases two eggs on her own
  • due to fertility pills or shots that make a woman release more than one egg
  • because we put 2 (or more) embryos in the uterus
  • because an embryo splits after it is in the uterus


The first three are dizygotic/fraternal twins and the last is monozygotic/identical twins.  Most twins are fraternal.


Since everyone wants them why don’t we put more embryos in, or use higher doses of fertility medications, to try and get them?  Risk.  We want HEALTHY twins, but twin pregnancies are riskier so the chances of a healthy child decrease with twins.  Here are the risks:


  • mother developing diabetes
  • mother developing high blood pressure
  • delivery early (pre-term birth)
  • babies spending time in ICU
  • more c-sections for delivery
  • higher rates of cerebral palsy
  • 6% chance of losing one twin before birth

Ref: UptoDate 2018


In talking to patients I will typically say 95% of our singleton babies are both healthy at term, but for twins it’s 85-90%.  So trying for one baby at a time improves the chances you’ll have a healthy baby.  As a physician my promise is to “first do no harm.” So encouraging singleton pregnancies is the most responsible practice.  That being said, I recognize in many instances taking a risk with two or three embryos might make sense to optimize the chances of one baby. It’s a balance.


I was reading an interesting paper last night, published this past month in Fertility & Sterility, the examined cases of twins when just one embryo was put into the uterus.  After single embryo transfer in the US between 2010 and 2013 1.7% were twins.   Most of them are the result of spitting of the embryo (identical twins) but some were dizygotic/fraternal.  Out of 1000 single embryo transfers, then, 17 were twins and 3 of those 17 were dizygotic/fraternal.   What?   How?  The woman must have conceived on their own in the same cycle.  She would have had to release an egg and it been fertilized with sperm and implant around the same time the other embryo was being put into the uterus.  She can release en egg in a fresh IVF cycle if not all eggs are removed during the egg retrieval and she then ovulates, or during a frozen cycle she ovulates despite the preparatory estrogen.


We had such a case recently and the woman was in complete disbelief.  She certainly thought I was a mistaken or unskillled at ultrasound!  Who knows, but after 2 more scans she came to understand that she was one of those 3 per 1000 single embryo transfers who get fraternal twins.  


Twins are exciting and scary all at once. If things go well, they go well and you have two healthy babies. Ultimately one at a time is safer for mom and baby but sometimes even when you plan for one you can still get two!


Reference: Vega M, Zaghi S, Buyuk E, Jindal S. Not all twins are monozygotic after elective single embryo transfer: analysis of 32,600 elective single embryo transfer cycles as reported to the Society for Assisted Reproductive Technology.  Fertil Steril. 2018 Jan;109(1):118-122.



Dr. Beth Taylor MD, FRCSC
Reproductive Endocrinology & Infertility