**5.3. Previous C-section**

When the second twin is in a non-cephalic presentation, vaginal delivery is controversial. Some studies say that neonatal morbidity is higher for the second twin in those cases and an elective cesarean section should be planned [16, 17]. However, both a systematic review and meta-analysis [14] and a recent published prospective cohort study [18] support that cesarean deliveries neither add neonatal morbidity nor mortality. Therefore, a vaginal delivery is a safe option. In those cases, the second twin can either be delivered by breech extraction or an

Finally, when the first twin is non-cephalic, the safest delivery mode is the cesarean section. A randomized multicenter trial, The Breech Trial, showed that a planned cesarean delivery decreases significantly perinatal mortality and neonatal serious morbidity, when compared with a planned vaginal delivery in pregnancies with a non-cephalic presenting

Twin pregnancies are more likely to show deviations in fetal growth curve. Conditions such as prematurity, intrauterine growth restriction and fetal malformations are common in multiple gestations, raising the risk of mortality and perinatal morbidity to 3–7 times when compared to single pregnancies [21]. Prematurity is present in approximately 55% of twin pregnancies,

When comparing the weights of fetuses from twin pregnancies to those of single pregnancies, it is observed that fetuses of twin pregnancies have a lower weight than fetuses of single pregnancies, especially from the end of the second trimester. It is known that this variation between the weights starts at around 28 weeks and at 38 weeks the 50th percentile for a twin pregnancy corresponds to the 10th percentile for a single pregnancy [23], but this difference does not seem to increase neonatal mortality. Therefore, it is argued that the lower weight of twin fetuses, when compared to that of single pregnancies, may be physiological of this

Accuracy in the estimation of fetal weight is of paramount importance for the proper followup of prenatal care and ultrasonography study has been the main tool for this evaluation.

Currently fetal weight estimation by ultrasonography is most commonly performed by the formula of Hadlock et al. [24], which uses two-dimensional measures of cephalic pole, abdominal circumference and femur length. However, studies have shown that formulas using two-dimensional parameters can generate variations of up to 15% in relation to the real

New methods have been sought to improve the accuracy of fetal weight estimation such as three-dimensional ultrasonography. In the early 2000s, Lee et al. [26] introduced a new sonographic parameter, the fraction limb volume. This parameter is based on evaluation of 50% of

This method has the advantage of reducing the time spent to perform the test, maintaining a

external cephalic version can be attempted [19].

**5.2. Fetal weight estimation in twin pregnancies**

with adverse consequences even in short and long term [22].

twin [20].

214 Multiple Pregnancy - New Challenges

condition.

weight of the fetus [25].

bone diaphysis length (arm and thigh).

good accuracy for the estimation fetal weight.

A previous cesarean delivery is considered a risk factor for an emergency C-section after attempting a vaginal delivery in twin pregnancies [34]. Regardless, a caution trial of labor can be a safe option in those patients, when the first twin is cephalic [35].

On the other hand, patients with two or more previous cesarean sections should not attempt a vaginal delivery due to higher risk of uterine rupture.
