DISCUSSION
In this study of 424 pairs of consecutive GDM pregnancies, an ADO or ANO
in the index GDM pregnancy conveyed a greatly increased risk of the same
outcome in the subsequent GDM pregnancy. While these risks have been
described in the general antenatal population, they have not been
previously quantitated in GDM.
Compared to index GDM pregnancies, the rates of instrumental delivery
and emergency Caesarean section were more than halved in subsequent
pregnancies, with correspondingly increased rates of elective Caesarean
section, lower rates of vaginal delivery and induction of labour. This
could be explained by a greater consideration of elective Caesarean in
women with a previous Caesarean section, and also be influenced by a
history of ADO or LGA in the index pregnancy. But while ADOs were
improved in subsequent pregnancies, the risk of having an ADO was still
far greater in women with a history of the same ADO in the index
pregnancy, with a RR of 3.09 for instrumental delivery and 2.20 for
emergency Caesarean. These risks may justify a lower threshold for
elective Caesarean in women with recurrent GDM and history of
instrumental delivery or emergency Caesarean.
While delivery outcomes were improved in subsequent GDM pregnancies, ANO
rates were unchanged in index versus subsequent pregnancies, with LGA
rates of ~16%, SGA rates of ~8% and
overall composite ANO rates of ~26%. Given that women
with subsequent GDM pregnancies were older, had a higher BMI and were
more likely to require medication, it could be hypothesised that they
should have had a higher rate of adverse outcomes, which was not the
case. One explanation could be that women were diagnosed earlier due to
earlier screening which may have affected ANO rates.
Three retrospective studies have examined the comparative rates of LGA
in first and second GDM pregnancies(10-12), although
none have analyzed detailed individual-level data across consecutive
pregnancies. Similar to our study, a study of 389 predominantly
Mexican-American women found no difference in rates of LGA(10) with a 15.6% rate of LGA in the first GDM
pregnancy compared to 19.9% in the second GDM pregnancy. Another
retrospective study of pregnancy-pairs (12). also
found that LGA and SGA rates were not significantly different between
pregnancies with first time GDM and those with recurrent GDM. However,
the authors also did not quantitate the risk of recurrent adverse
outcomes from one GDM pregnancy to the next.
Only one study has quantitated the risk of recurrent adverse outcome in
recurrent GDM pregnancy pairs(11). In contrast to our
study, the LGA rate increased in the second GDM pregnancy (22.4% vs
13.8%, p < 0.05). The risk of recurrent LGA was 55.7%,
comparable to our rate of 45.1%. While this population-based study had
a large of number of pregnancy pairs, it did not examine other ANOs such
as SGA and fetal or neonatal death, and detailed clinical information
such as timing of diagnosis of GDM, medications, results of the GTT,
maternal BMI and interpregnancy interval, were not included in the
analyses. Thus, they were not able to evaluate for effects of other
factors associated with increased risk of recurrent LGA.
Our study lends a new perspective by tracking the incidence of ANOs in
individual women over consecutive GDM pregnancies. The RR of repeat
outcome for women with LGA, SGA or any ANO in their GDM pregnancy was
4.5 fold, 5.0 and 2.1 respectively. Put another way, nearly half of
women with LGA, 70% of women with SGA and 44% of women with the
composite ANO in their subsequent pregnancy had the same outcome in
their index pregnancy. Thus while ANO rates were similar in index and
subsequent pregnancies as a group, a substantial proportion of adverse
outcomes were occurring in the same women. Of additional interest is the
very low risk of SGA in women with prior LGA, and the low risk of LGA in
women with prior SGA.
Multivariate analysis of ANO in the subsequent pregnancy showed that
having the same outcome in the index pregnancy was by far the strongest
risk factor, with a 3.1-fold for LGA, 4.7-fold for SGA and 2-fold for
the composite ANO. In LGA, this risk far outweighed that of maternal
BMI, a well-established risk factor for LGA (13, 14).
In SGA, this risk far outweighed that associated with increasing
interpregnancy interval.
The risks of recurrent LGA or SGA have not been previously described in
GDM. Our calculated risks are similar in magnitude to the five-fold risk
observed in general obstetric cohorts for LGA (15)(16) as well as for SGA (17, 18).
Moreover, severity of GDM based on the GTT results, need for medication
and timing of diagnosis were not associated with subsequent pregnancy
outcomes. While on the surface these data may suggest that GDM does not
impact greatly on risk of recurrent ANOs, we were not able to include
the modifiable factors of gestational weight gain and a measure of
glycaemia such as HbA1c in our model, and cannot discount the importance
of weight and glycemic management.
Limitations of this study include unavailable data on maternal smoking
status, gestational weight gain and hypertensive disorders. There were
slight differences in glucose targets between centres and slight
differences in population demographics although reassuringly,
interpregnancy changes did not differ between centres. We relied on
medication requirement as a surrogate marker of glycaemia. It is
possible some of the participants may have had undiagnosed type 2
diabetes.
Strengths of this study included the inclusion of consecutive GDM
pregnancy pairs and the analysis of longitudinal data in individual
patients that allowed us to assess risk of adverse outcomes in the
context of previous complications. We were able to adjust for relevant
clinical covariates including interpregnancy duration and interpregnancy
weight gain, both pertinent to recurrent GDM. The definitions of LGA and
SGA were based on customised centiles for an Australian population.
According to current standards, diagnostic criteria, glucose targets and
weight gain targets are applicable to all GDM women, irrespective of
their history of ADO/ANO. Given the high frequency of repeat
complications, should these criteria be tightened for women with prior
LGA, or relaxed for women with a history of SGA? Would early detection
of GDM be effective in prevention of LGA, or could it be detrimental in
those with prior SGA? Further studies are required to provide
evidence-based guidelines for managing recurrent GDM.