There is a consensus [1] that the majority of twin births results from natural conception (60 %), with assisted reproductive technologies (ART) treatments accounting for 8 % to 16 %, and ovulation induction accounting for the remainder. However, Pinborg et al. [2] note that, in Denmark, for example, one-third of twin pregnancies are a result of in-vitro fertilization (IVF), including intracytoplasmic sperm injection (ICSI) treatment.

Several factors contribute to the increased incidence of multiple gestations resulting from infertility treatments: competitive pressures to apply ovulation induction or IVF early to achieve high pregnancy rates; economic pressure on patients restricting the number of ART cycles they can attempt (as an example, the Portuguese state pays for only 3 cycles); and pressure from infertility couples to transfer more than one embryo, to improve the chances of pregnancy or to obtain two or more babies with a single treatment.

Multiple pregnancies, when compared with singleton gestation, have an increased risk of maternal complications, such as hypertensive disorders, diabetes and preterm delivery [3]. Due to the risk associated with multiples, there has been a global effort to reduce multiple pregnancy rates to a minimum while maintaining an acceptable level of successful IVF pregnancy rate.

Elective single embryo transfer (eSET) is advocated in several European countries. However, despite clinical recommendations and policy statements, patients in clinical practice frequently do request the transfer of multiple embryos in order to have twins [4], a full family resulting from a single treatment. Such requests conflict with policy guidelines and create an ethical dilemma for physicians: should the physician respect the autonomy of the patients, or follow policies intent on protecting the mothers and their children? [4] Posing the question in a different way, what is the likelihood that a twin pregnancy will end with two healthy babies? The main problem is that the chances of having a newborn with a weight <1500 g is 10 times greater in twin pregnancies compared to singletons [5], and at least 50 to 60 % of all twins are born before 37 weeks. In great part due to preterm delivery, the mean cost per twin pregnancy is significantly higher when compared with singleton pregnancy [6].

In Portugal, in 2018, two embryos were transferred in 55.7 % of the cases [7]. It is therefore important to understand twin pregnancy and find risk factors for adverse outcomes. Obstetric history could help, as it is known that a previous preterm singleton increases the risk of delivering a preterm twin from 21 % to 57 % [8]. However, nulliparous women do not have an obstetric history, and yet they are often those who seek infertility treatments. If a couple asks for a double embryo transfer, might maternal anthropometric measurements help clinicians determine which women are at lower risk of complications during pregnancy if pregnant with twins?

Short stature is associated with significantly increased risk of very early preterm birth and early preterm singleton and twin births in pregnancies resulting from IVF [9]. A paper from China shows us that about 47.8 % of women with multiple gestation experienced pregnancy complications; these women were more likely to be shorter than women who did not experience complications [9]. On the other hand, Tudela et al. [10] found that maternal short stature ≤159 cm was not associated with preterm birth, fetal growth restriction, or cesarean section rate. To clarify this subject, we evaluate data from twins followed at our institution during the period 1995–2020.

Our question is: Should maternal height in nulliparous women be taken into account when opting for double embryo transfer (DET) in ART treatments?