Information for Medical Practitioners from the Institute for Clinical Evaluative Sciences
M ARCH 2000 V OL 6 N O 2
using research in your practice
To Maternity and Beyond…
In some areas of antenatal care,
the literature is clear; in others,
many questions still remain.
Here is a review of some recent
research and recommendations...
Maternal serum screening (MSS) is a
non-invasive method of estimating a
woman’s risk of having a fetus with
Down syndrome (DS), trisomy 18 or
open neural tube defects (ONTD).
MSS is ideally performed at 16 weeks
gestation (range 15 to 20 weeks). It is
not useful in multifetal pregnancies. J. Sherman
Traditionally, prenatal screening for DS
has been limited to offering invasive from the Ministry of Health in Ontario women who will be 35 at the estimated
testing (amniocentesis or chorionic recommends that, if dates are unclear, time of delivery be offered MSS and
villus sampling) to women who will a dating ultrasound prior to MSS should remain eligible for amniocen-
be 35 or older at the estimated time should be performed. Women with a tesis regardless of the results.
of delivery. Women are also offered calculated probability exceeding the
testing if they or their partners have a
previous child with a chromosomal
cut-off should be offered a dating
ultrasound, if not already done. If, on
abnormality or are carriers of a chromo- the basis of accurate dating, the risk Sampling Versus
somal rearrangement. However, the
vast majority of DS babies would not
still exceeds the cut-off, the woman
should be offered genetic counselling
be detected antenatally by using age and amniocentesis. In one study, the Amniocentesis for fetal karyotyping
screening as they are born to women number of cases of DS detected per has traditionally been done after 15
under the age of 35. By identifying amniocentesis performed was 1/87. weeks gestation because relatively large
women with a risk of at least 1/385 amounts of amniotic fluid (up to 20 cc)
(the DS risk in a 35 year old at term) Researchers continue to work on can be aspirated without significant
and offering them amniocentesis, improved methods for the detection of technical problems. This amount of
approximately 70% of fetuses with DS chromosomal anomalies. Current fluid is desirable to minimize the
can be detected. MSS can also detect areas of research include nuchal
...continued on page 3
approximately 85% of trisomy 18 translucency, urine beta-core fragment
cases and 80 to 85% of ONTD. and umbilical Doppler velocimetry.
At this time, however, the Society of
There is, however, a significant false Obstetricians and Gynaecologists of INSIDE
positive rate, which ranges from Canada (SOGC) recommends that, Editor’s Letter . . . . . . . . .2
3 to 7%. The most common reason where feasible, MSS screening be Nutrition in Pregnancy … 6
for a false positive result is inaccurate offered to all pregnant women in The ALPHA Project . . . . .7
gestational age. The MSS protocol Canada. They also recommend that Maternity Care Calendar .8
March 2000 Vol 6 No 2 informed
To Maternity and Beyond… At 18 weeks, there is sufficient fetal (sex-industry workers, women with
(continued from page 1) development to facilitate the detection multiple sexual partners, intravenous
of anomalies by ultrasound while allow- drug users) be counselled about
ing time for additional information to reducing high-risk behaviour and
be obtained and discussed with the offered retesting in 6 months, if the
pregnant woman (couple) to deter- initial test result is negative (Grade B).
mine available options. An 18 week
ultrasound can also establish gestational To prevent transmission from mother
age to +/- seven days, determine to child, seropositive women and their
placental location and lead to earlier infants should be offered treated with
detection of multiple gestations. zidovudine. Breast-feeding should be
chance of the laboratory not having
enough viable fetal cells to culture. A
The Canadian Task Force on Preventive
major disadvantage of later amniocen-
tesis, however, is that the final result is
Health Care found no evidence of
improved perinatal mortality or morbid-
usually not available until after 18 weeks,
a delay which can be distressing to the
ity when serial ultrasound screening in Diabetes Mellitus
the second and third trimesters was
parents. Newer methods of analyzing Gestational diabetes mellitus (GDM)
done in women without clinical
the amniotic fluid have been developed occurs in 2 to 4% of all pregnancies.
indications. There is no scientific
allowing for a more rapid screen, but The established morbidity for the baby
evidence of a deleterious effect from
these tests are not available everywhere. includes macrosomia and neonatal
diagnostic ultrasound on the fetus.
hypoglycemia. The pregnant woman
Earlier options include chorionic villus with GDM is at later risk for postpartum
sampling (CVS) and early amniocen- diabetes mellitus, impaired glucose
tesis. CVS is usually performed tolerance and lipid abnormalities.
between 10 and 12 weeks gestation. Usually at 24 to 28 weeks gestation,
The Cochrane Collaboration reviewed women are given a 50 gm oral glucose
the literature and found that second load to screen for GDM. Referral for
trimester amniocentesis appears to be further oral glucose tolerance testing
safer than CVS. More women needed (OGTT) is warranted if the plasma
a second diagnostic test with CVS due glucose level one hour post ingestion
to sampling and technical failures, and is greater than 7.8 mmol/L. If the
there were more false positive and false Screening for HIV level at one hour is greater than 10.3
negative results. Miscarriage was also mmol/L, GDM can be diagnosed from
more common in CVS (1 to 1.5% vs The transmission rate of HIV from an the 50 gm challenge (see table).
0.04 to 0.8%). In addition, CVS cannot HIV-positive mother to her child during
detect ONTD. The increase in miscar- labour and delivery is 12 to 30%. In 1998, the Canadian Diabetes Associa-
riages after CVS, when compared to In addition, about 14% of uninfected tion published clinical practice guide-
amniocentesis, appears to be procedure- infants who are breast-fed by HIV- lines on the management of diabetes,
related. Therefore, earlier diagnosis positive mothers will become infected. which included a recommendation
and its benefits must be weighed A multicentre study showed a dramatic that all pregnant women should be
against the greater risk of miscarriage. reduction in mother-to-child transmis- screened for diabetes, unless they are
sion of HIV from 25 to 8%, following
...continued on page 4
Early amniocentesis (done between zidovudine treatment.
9 to 14 weeks gestation) is generally not
offered, as it is associated with a greater In 1998, a Canadian Expert Working DIAGNOSIS OF GDM
risk of spontaneous miscarriage and Group on HIV Testing recommended GDM is established if plasma glucose
clubfoot than with CVS. that all pregnant women in Canada be levels (mmol/L) are:
counselled about HIV infection and 1. After 50 gm challenge, at 1 hour: >10.3
Prenatal Ultrasound offered HIV testing (Grade B: fair evidence 2. After 75 gm OGTT, two of the following
three values are met or exceeded:
to include in clinical practice). In their
In a recent policy statement, the SOGC literature review, they found that ✔ Fasting >5.3
✔ 1 hour >10.6
recommended that all women be targeted testing of only pregnant
✔ 2 hour >8.9
offered an ultrasound centred at 18 to women at high risk for HIV failed to (If only one value is met or exceeded, the
19 weeks gestation, after appropriate identify a substantial proportion of diagnosis is impaired glucose tolerance of
discussion about the benefits, limita- HIV-positive pregnant women. They
tions and safety of the examination. recommend that women at high risk Adapted from: CMAJ 1998;159(8Suppl):S1-29.
An ultrasound at 18 weeks can reliably determine placental location.
Adapted from SOGC Policy Statement No. 78, August 1999
informed March 2000 Vol 6 No 2
To Maternity and Beyond… universal screening for high-risk GBS are transmitted to 40 to 70% of
(continued from page 3 ) women with lower plasma glucose the newborns of colonized mothers,
in a very low-risk group. Low-risk thresholds (see table). These new only 1 to 2% of these babies develop
people include lean Caucasian women strategies resulted in an almost 35% disease. Early onset disease (<7 days of
under the age of 25 years, with no reduction in the number of screening age) is more common than late-onset
personal or family history of diabetes tests performed and detected 82% of disease (7 days to 3 months of age)
and no history of large babies. women who had gestational diabetes, and is associated with higher mortality.
as compared to 78% through usual
care. There was also a small decrease Until further studies are available, the
Estimating the Risks for SOGC recommends that identification
in the percentage of false positive
Developing Gestational Diabetes and management of women whose
Risk Factors Score newborn infants might be at increased
✔ Age If health care providers choose not to risk of GBS are acceptable by either of
≤ 30 yrs 0 assess each individual’s risk of diabetes, two methods:
31 to 34 yrs 1 it may be prudent to screen all preg-
1. Universal screening of all pregnant
≥ 35 yrs 2 nant women with the 50 gm glucose
women at 35 to 37 weeks gestation
≤ 22.0 0 with a single combined vaginal-
22.1 to 25 2 anorectal swab and the offer of
≥ 25.1 3 intrapartum chemoprophylaxis to
✔ Race all GBS-colonized women.
2. No universal screening but intra-
Asian 5 partum chemoprophylaxis for all
Other 2 women with identified risk factors.
This strategy should also be used in
Risk Factors Recommendations
cases where universal screening is
Low Risk= 0-1 • No screening
the policy but was not done or the
Intermediate • Screening with threshold test results are not available.
Risk= 2-3 of 7.8 mmol/L
High Risk= ≥4 • Screening with lower For intrapartum chemoprophylaxis, the
threshold of 7.1-7.2 mmol/L
SOGC recommends using intravenous
Adapted from: NEJM 1997;337(2):1591-1596.
Prevention of Early- ampicillin (2g initially followed by 1 to
2g every 4 to 6 hours) or penicillin G
Investigators at the University of onset GBS Infections (5 million units every 6 hours) until
Toronto have developed another in Newborns delivery or labour is stopped. Women
who are allergic to penicillin may be
screening strategy, which estimates
pregnant women’s risks of gestational In 1997, the SOGC issued a policy given intravenous clindamycin
diabetes on the basis of their clinical statement on group B streptococci (300 to 600 mg every 8 hours.) s
characteristics. The study, published (GBS) infections, a major cause of
in the New England Journal of Medicine, newborn morbidity and mortality. The Bottom Line
categorized women’s risks on the basis The source of the infection is usually
of age, race and body-mass index vaginal, transmitted to the infant s Where a screening program is
(BMI) before pregnancy. The new available, MSS screening should be
either before or during the process of offered to all pregnant women.
strategy, developed and validated in birth. GBS are rarely transmitted to
3,131 pregnant women, divided the fetus prior to labour or rupture of s Second trimester amniocentesis is
safer than chorionic villus sampling.
women into three groups: low-risk membranes. GBS colonization rates
(requiring no screening manoeuvre), are estimated to be between 15 to 40% s All pregnant women should be
offered a screening ultrasound at
intermediate-risk (usual care) and among pregnant women. Although 18 to 19 weeks gestation.
s All pregnant women should be
counselled about HIV infection and
GBS Risk Factors for which Intrapartum Chemoprophylaxis is Recommended offered HIV testing.
✔ Term labour ✔ Pre-term labour
(≥37 weeks gestation), and (<37 weeks gestation) s All pregnant women should be
offered screening for GDM, unless
• Prolonged rupture of membranes. ✔ Previous delivery of a newborn with GBS
Chemoprophylaxis should be given if labour
they are in a low-risk group.
disease regardless of current GBS
and/or ruptured membranes are likely to colonization status s Strategies to reduce the risk of GBS
continue beyond 18 hours (neonatal benefits
✔ Previously documented GBS bacteriuria disease in newborns include
are optimally achieved if antibiotics are
given at least 4 hours prior to delivery).
universal screening and intrapartum
Source: SOGC Clinical Practice Guidelines. Policy Statement. chemoprophylaxis for all women
• Maternal fever during labour No. 61, June 1997. with identified risk factors.