When and How to Deliver Twins?
I. Blickstein
Dept. of Obstetrics and Gynecology, Kaplan Medical Center, Rehovot and the
Hadassah-Hebrew University School of Medicine, Jerusalem, Israel
Summary
This presentation discusses two controversial issues of twin birth:
timing and mode of delivery. There are several lines of evidence to
suggest that twins may benefit from induced delivery after
completing 38 weeks. However, there is no prospective randomized
trial to demonstrate this beneficial effect. Similarly, there are several
arguments to support the idea that all or most twins should be
delivered by cesarean section, although there is little evidencebased support of this view.
Introduction
Twins and higher-order multiple pregnancies have reached epidemic
magnitudes in most developed countries. The major contributors to this
epidemic, namely assisted reproduction did not arise ex vaccuo. The
availability of effective contraception on one hand and effective fertility
treatment on the other hand helped women to plan motherhood. It follows
that when pregnancy is desired, usually after completing other stages in
life, these women are at an advanced age, which by itself is an accepted
risk for natural multiples. In addition, reduced fecundity of older age,
significantly increases the need for treatment. Thus, social trends and
available effective therapy act in concert to increase the risk of multiple
birth. Population-based trends showed that the ratio of induced to
spontaneous twins increased since the ‘70s from 1:40-50 in to 1:2-3 twins.
Obviously, these trends are accentuated in centers with busy infertility
clinics. [1]
When twin delivery was expected once in every 80 births, the
contribution of multiple pregnancies to the overall cesarean rate
was negligible. However, with the current figures of multiple births,
there is little doubt about their contribution to the cesarean birth rate
and its associated morbidity and mortality is significant. [2] This
presentation discusses two controversial issues of twin birth: timing
and mode of delivery.
When to Deliver Twins ?
In the usual setting, the major problem associated with twins is preterm
birth rather than term or post-term births. While “term” is defined by the
appropriate period during pregnancy when most pregnancies end by
spontaneous labor, this period is not defined in twins. Because 70 to 80%
of all twins do not reach “term” as defined for singletons (i.e., 38-42
weeks), the question of whether “term” in twins occurs earlier raises two
direct concerns. First, whereas it is obvious that all preterm twins will be
also preterm by singleton standards, it is unknown until which gestational
week the risks of preterm birth in twins are similar to those in singletons.
Second, as “term” may occur earlier in twins than in singletons, twins, if
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carried to the singleton-“term”, may be exposed to unnecessary risks of
post-“term” pregnancy. There are several lines of evidence to support the
concept that “term” in twins occurs before 38 weeks’ gestation.
Statistical
The distribution of twin births by gestational age is almost identical to that
of singletons, but shifted towards a lower gestational age. The ‘tails’ of the
distribution, which correspond to deliveries before and after the mean
gestational age at birth of singletons, should also correspond to those in
twins. Data from the USA suggest that this is indeed the case: 18% of
singletons and 18% of twins were delivered after 40 and 37 weeks,
respectively. [3] It is expected that pathologies related to the period
defined by the upper ‘tail’ in singletons will be at least similarly frequent
in twins.
Growth
Normal twin growth patterns show accelerated growth before deceleration
begins towards the end of gestation. Comparison between growth patterns
of twins and singletons suggests that incremental growth reaches a plateau
earlier in twin pregnancies. This pattern suggests that any additional
week(s) beyond a, at about 37-38 week’s gestation does not significantly
increase birth weight.
Morbidity
Luke et al. [4] compared total birth costs in twins and in carefully matched
singleton controls to assess the overall morbidity associated with plurality
and gestational age. Preterm births rather than plurality characterized the
predominant cost factor. Compared with singletons, twins experienced
increased morbidity and associated costs after 38 week’s gestation.
Several studies stratify the frequency of cerebral palsy in twins versus
singletons by gestational age. The most significant risk of cerebral palsy in
all babies was prematurity. However, whereas the risk of cerebral palsy in
singletons and in twins decreases steadily until 36-37 weeks, the risk of
cerebral palsy in singletons continues to decrease but that for twins
increases once again after 37 weeks’ gestation. [5]
Mortality
Minakami and Sato [6] as well as others, showed that the incidence of
perinatal death in multifetal pregnancies (predominantly twins) gradually
declines until 37 to 38 weeks' gestation and then increases again, as
compared with 39 weeks' gestation in singletons. The lowest incidence of
perinatal death seen at 38 weeks' gestation in multifetal pregnancies was
similar to that seen at 43 weeks in singletons. These authors concluded that
fetuses of multifetal pregnancies are at an increased risk of death after 3738 weeks.
These lines of evidence are sufficient to indicate that “term” occurs
earlier in twins than in singletons. Accordingly, limiting the
estimated date of delivery to 37 - 38 weeks may be appropriate in
twin pregnancies. It follows that there is some merit in delivering
twins who completed 38 weeks. Regrettably, there are only few
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published series on this issue. One recent study [7] tried to
determine the gestational age at delivery associated with the lowest
rates of perinatal mortality, respiratory distress syndrome, and long
hospital stays among twins. The nadirs of perinatal mortality rate,
respiratory distress syndrome incidence, and long hospital stay rate
were seen at delivery dates of 39, 40, and 38 weeks' gestation,
respectively. Pairs delivered vaginally without the induction of labor
had the lowest perinatal mortality rate at 37 weeks' gestation. The
authors concluded that induction of labor should be routinely
considered for twins at 37 to 38 weeks' gestation.
The overdistended uterus of twin gestation, however, is a relative
contraindication for labor induction and therefore pregnancy is often
terminated by cesarean birth. Recent studies have suggested that
pre-induction ripening of the cervix is both effective and safe. [8-9]
Unfavorable cervical conditions seem to be no longer an obstacle
for vaginal birth in appropriate candidates. Nevertheless, labor in
twin gestations that was induced, rather than spontaneous, required
more time, more oxytocin, and was associated with a higher
cesarean delivery rate. [10]
It should be stressed that if 38 weeks or more for twins represent
“post-term”, it does not mean that 36-37 weeks are equivalent to
term in singletons. Although twins probably gain neurological
maturity at this age, the risk of respiratory distress is not eliminated.
Indeed, twins with respiratory disorders were more likely to have
been delivered by cesarean section between 36 and 38 weeks'
gestation. [11] The authors concluded that without a clear indication
for delivery, waiting until labor or until after 38 weeks' gestation
should be considered.
How to Deliver Twins?
Assuming that about half of the twins are delivered anyway by
cesarean section, the contribution of twins to the overall cesarean
rate can be estimated. With increasing cesarean rates, the
contribution of twin to that rate becomes negligible. Performing a
cesarean for all twins is expected to increase the overall rate by
10% in a service with an overall 10% cesarean rate but will add only
3.3% to a service with an overall rate of 30%. [2]
Any decision about the mode of delivery that includes probability
estimation is prone to serious confounding. An incomplete list of
confounding variables in deciding about the mode of delivery in
twins is shown in Table 1. It seems that one may find an indication
for cesarean birth in almost every twin pregnancy. However,
research has not quantified the attributed risk of most of the
confounding variables listed in Table 1 and has taken the easy way,
by focusing on presentation and size to deal with the optimal mode
of twin birth.
Vertex-vertex pairs are unanimously considered appropriate
candidates for vaginal birth, with few exceptions related to size
64 Blickstein
and/or gestational age. Vertex-nonvertex are considered
conceivable candidates for vaginal birth, with many exceptions
related to size and/or gestational age. [12] The common exceptions
include small second-breech and dorso-inferior transverse lying
second twin. These situations demonstrate the skills of the
accoucheur to chose between external and internal versions.
Bedside sonography will guide the operator and minimize needless
blind ‘explorative’ interventions.
Nonvertex-vertex and nonvertex-nonvertex are generally
considered as an indication for abdominal birth. Recently, we
conducted a multi-center collaboration of a large sample of breechfirst twin pairs delivered in 13 European centers. [13] The data
indicated that when breech-first twin twins weighed <1500 g, there
was a 2.4-times higher risk of depressed (<7) 5-min Apgar scores
and a 9.5-times risk for neonatal mortality in vaginal compared with
cesarean birth. However, cesarean birth did not improve outcome
when breech-first twin weighed >1500 g. Importantly, no case of
“locked twins” has been encountered. The simple rule-of-thumb
indicates that the same criteria for singleton breech delivery should
be applied for twin breech delivery.
Conclusions
The current epidemic dimensions of multiple pregnancies translate
into true challenges for the obstetrician. Some are undoubtedly
controversial and represent clinical dilemmas. There are several
lines of evidence to suggest that twins should be delivered after
completing 38 weeks. Yet, there is no prospective randomized trial
that clearly demonstrate the anticipated beneficial effect. Similarly,
there are several arguments to support the idea that all twins should
be delivered by cesarean section, although there is little evidencebased support of this view.
References
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Lancs, 1999: 368-73.
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Table 1. Confounding variables in the decision for a cesarean birth in
twin pregnancies.
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