The rate of twin pregnancies has increased over the last decade in connection with assisted reproduction treatments. Twin pregnancies carry a higher risk for both the mother and the baby.
We frequently meet patients who are excited by the idea of a twin pregnancy. This situation requires us, as fertility specialists, to clearly explain the disadvantages of multiple gestation, with special emphasis on the risk of prematurity among other obstetric complications.
Nevertheless, although we always try to guide patients toward single-embryo transfer (SET), in patients without associated risk factors and in good health we may agree to the transfer of two embryos (DET) if, after understanding the implications, this is their choice.
What is the probability of twins with assisted reproduction?
Depending on the technique used, the cause of multiple pregnancy will differ.
Twin or fraternal twin pregnancy with spontaneous conception
The probability of a spontaneous fraternal twin pregnancy is 1.1%, while the percentage of identical twins is less than 0.5%.
Twin or fraternal twin pregnancy with artificial insemination (AI)
During AI, low doses of follicle-stimulating hormone (FSH) are administered to stimulate the growth of one or two follicles. In this way, at the time of insemination, sperm may fertilize two eggs and result in a twin pregnancy.
To reduce the risk of a triple or higher-order pregnancy, if three or more follicles develop we will cancel the cycle and advise the patient to use contraception if she has intercourse.
The overall twin pregnancy rate after AI is 9.2%.
Twin (or Fraternal Twin) Pregnancy with In Vitro Fertilization (IVF)
Except for rare cases of blastocyst splitting (an embryo that is 5 days old or older), twin pregnancies in IVF are typically the result of transferring two embryos.
Spanish legislation allows the transfer of up to three embryos (Law 14/2006). However, this upper limit is not standard practice.
The overall twin pregnancy rate in IVF cycles is 9.4%. Nevertheless, in good-prognosis patients, transferring two embryos carries a 42% risk of multiple pregnancy.
What is the difference between fraternal twins and identical twins?
Broadly speaking, fraternal twins result from the simultaneous fertilization and implantation of two different embryos produced by the fertilization of two separate eggs with two different sperm cells.
Identical twins, on the other hand, arise from the division of a single embryo that originates from the fertilization of one egg by one sperm cell. Because of this division, both twins share the same genetic makeup.
As an interesting point, the timing of the embryo’s division determines the type of twin pregnancy:
- Before day 3: A dichorionic–diamniotic pregnancy develops. Each twin has their own sac and placenta. This is the most common subtype, with an incidence of about 25%.
- Between days 3 and 8: Monochorionic–diamniotic twins develop — one placenta and two sacs.
- After day 8: Monochorionic–monoamniotic twins develop — one placenta and one sac for both fetuses.
Do assisted reproduction techniques increase embryo splitting?
In cases where there has been manipulation of gametes — such as intracytoplasmic sperm injection (ICSI), assisted hatching, or perforation of the zona pellucida for blastomere biopsy — the incidence of monozygotic twinning is estimated to be 2–12 times higher than in spontaneous conceptions (0.9%).
Other factors that may increase the incidence of twin pregnancy
- Advanced maternal age
- Family history (heredity)
- Race: Higher rates of twin pregnancies are observed in African populations.
- Maternal weight: A BMI >30 favors the release of more than one egg during ovulation.
A twin pregnancy is undoubtedly a unique experience and is often celebrated with great joy. However, as physicians, our primary goal is always a healthy baby and a healthy mother, which is why we strive to minimize risks and recommend single-embryo transfer or singleton pregnancy whenever possible.
Book a free consultationDo you have any questions or need more information?
At Fertility Madrid, we specialise in treatments such as egg donation, in vitro fertilisation and artificial insemination, among others.
Please do not hesitate to contact our professionals at our fertility clinic and they will answer all your questions without obligation.
