Preterm Birth in Singleton and Twin Pregnancies

Prematurity is a major contributor to neonatal morbidity and mortality. The incidence of preterm birth is higher in twin pregnancies than in singleton pregnancies: approximately 50% of twins are born before 37 weeks of gestation, accounting for 18%–25% of all preterm births. Physiological changes in pregnancy that strain maternal organ systems are more pronounced in twin pregnancies. Numerous maternal and fetal pregnancy-related complications that may lead to preterm delivery, such as hypertensive disorders of pregnancy, congenital malformations, and fetal demise, are more common in twin pregnancies than in singleton pregnancies. Preterm birth is a complex syndrome, and the underlying mechanisms may differ between singleton and twin gestations.

Tingleff T et al conducted this study to compare the distribution of pregnancy-related complications representing different pathways leading to extremely, very, and late preterm birth between singleton pregnancies and twin pregnancies. They also assessed the risk of spontaneous preterm birth in twin pregnancies compared with singleton pregnancies with adjustment for known risk factors.

For this population-based registry study, data were obtained from two national registers, the Medical Birth Register of Norway (MBRN) and Statistics Norway (SSB), comprising all births occurring in Norway between 1999 and 2018. Information was obtained from antenatal health cards filled in at checkups and hospital birth medical records.

Prevalence rates of pregnancy-related complications for extremely, very, and late preterm birth in twin and singleton pregnancies were calculated with 95% confidence intervals. The categories are defined as follows:

• Extremely preterm: <28+0 weeks of gestation
• Very preterm: 28+0–33+6 weeks of gestation
• Late preterm: 34+0–36+6 weeks of gestation

Preterm birth was significantly more prevalent in twin pregnancies than in singleton pregnancies in all categories: all preterm (54.7% vs 6.1%), extremely preterm (3.6% vs 0.4%), very preterm (18.2% vs 1.4%), and late preterm (33.0% vs 4.3%) births. Stillbirth, congenital malformation, and pre-eclampsia were more prevalent in twin pregnancies than in singleton pregnancies, but the prevalence of complications differed in the three categories of preterm birth. Pre-eclampsia was more prevalent in singleton than in twin pregnancies ending in extremely and very preterm birth. The adjusted odds of spontaneous preterm live birth were between 19- and 54-fold greater in twin pregnancies than in singleton pregnancies.

Preterm birth was more frequent in twin pregnancies (54.7%) than in singleton pregnancies (6.1%) of nulliparous women. Prevalence of pregnancy-related complications differed between singleton and twin pregnancies. Overall, preeclampsia was between three and four times more prevalent in twin pregnancies, but the role of pre-eclampsia as a contributor to preterm birth seemed to differ in singleton and twin pregnancies according to gestational age. Pre-eclampsia was more prevalent in singleton pregnancies resulting in extremely and very preterm births, but was more prevalent in twin pregnancies resulting in births from 34 weeks of gestation onwards. PROM was distributed equally among twin and singleton pregnancies leading to preterm birth. When obstetric complications associated with preterm birth were excluded from the analysis, the ORs for extremely, very, and late preterm birth in twin pregnancies compared with singleton pregnancies increased dramatically from the range of 8–15 to the range of 19–53.

Singleton and twin pregnancies seem to have different pathways leading to preterm birth. Stillbirth, congenital malformations, and pre-eclampsia were more prevalent in twin pregnancies than in singleton pregnancies in the entire population. However, pre-eclampsia was more prevalent in singleton pregnancies than in twin pregnancies ending in extremely or very preterm births. These findings indicate that hypertensive disorders play different roles in twin and singleton preterm births, probably because preterm twin births occur at an earlier gestation for other reasons. The role of twin pregnancy as a risk factor for preterm birth increased dramatically when removing other major risk factors for preterm birth. Tailored antenatal care should be offered to women expecting twins, including considering twin pregnancy a sufficient risk factor to offer prophylactic ASA. In addition, this study corroborates that single-embryo transfer is the appropriate strategy in artificial reproductive therapy.

Source: Tingleff T, Räisänen S, Vikanes Å, Sandvik L, Sugulle M, Murzakanova G, et al. Different pathways for preterm birth between singleton and twin pregnancies: a population-based registry study of 481176 nulliparous women. BJOG. 2022;00:1–9. https://doi.org/10.1111/1471-0528.17344