Why was the cohort set up?
The Cambridge Baby Growth Study (CBGS) is a prospective, observational pregnancy and
birth cohort with detailed assessments and sample collections in infancy and continuing
childhood follow-up. Women were recruited during early pregnancy from April 2001 to
March 2009; they attended several study visits throughout pregnancy, and then repeatedly
with their infants until the age of 2 years. The main aim of the study has been to
investigate the antenatal and postnatal determinants of infancy growth and reproductive
development, including environmental, genetic, hormonal and nutritional exposures.
Collection of extensive anthropometric, nutritional, demographic and biological data
has allowed detailed examination of the first 1000 days of life, a time of emerging
importance for the early origins of health and disease, where in utero exposures,
early growth, nutrition and genetic interactions are thought to contribute to later-life
disease risk,
1–3
All women were recruited from a single study site: the Rosie Maternity Hospital, Cambridge,
UK, and the Cambridge local research ethics committee approved the study. Initial
funding was from the European Union Framework V, with the specific objective to investigate
the effect of environmental factors during pregnancy on male offspring reproductive
development over the first 2 years of life. From the outset, the study was designed
to also include female offspring and to collect data on infancy growth and body composition.
Subsequent funding and collaborations have allowed further nested case control studies
and continuing follow-up through childhood. Cohort funding has been received from
the World Cancer Research Fund International, the Newlife Foundation for Disabled
Children, the Mothercare Group Foundation and a collaborative research grant from
the European Society for Paediatric Endocrinology. Support from the Medical Research
Council (MRC) has been awarded through a clinical research training fellowship (G1001995),
a Cambridge University MRC Centenary award, and assisted by MRC Department of Epidemiology
programmatic funding (U106179472). Lipidomic assays were also supported by the MRC
(UD99999906) and the Cambridge Lipidomics Biomarker Research Initiative (G0800783).
Genotyping and placental RNA work was funded by grants from the Evelyn Trust, the
Wellbeing of Women and Diabetes UK (11/0004241). Funding from Mead Johnson Nutrition
has allowed further biochemical analysis of biological specimens, in particular breast
milk samples, and childhood follow-up. Continued support and funding are provided
by the National Institute of Human Research Cambridge Comprehensive Biomedical Research
Centre.
Who is in the cohort?
Women were recruited during early pregnancy, at approximately 12 weeks of gestation,
at their first routine antenatal ultrasound clinic in the Rosie Maternity Hospital,
Cambridge, UK. At the time, the hospital had approximately 5000 deliveries per year
from a mixed urban and rural community mainly residing in South Cambridgeshire. Research
nurses introduced details of the study and distributed information sheets. Those who
showed interest in the study were followed up by telephone and the first study visit
was arranged, where informed written consent was provided. Exclusion criteria included
women under the age of 16 years, or those unable to give informed consent.
Figure 1 shows the numbers of participants accrued and followed up throughout the
study period: 2229 women were recruited in early pregnancy (∼ 50% of those who were
approached for participation) and 1658 offspring were studied (including 22 sets of
twins), following additional written consent from the mother on behalf of her infant(s).
Of these infants, 1183 were seen at 1 year of life and 1061 completed the infancy
study protocol at age 2 years.
Figure 1.
Participant numbers from recruitment and the infancy study protocol. Singlets, singletons.
Offspring who were followed until at least 1 year of age, and are now aged between
5 and 10 years, are currently being invited for a follow-up study visit. At the time
of writing, 423 children have been invited out of a total of 917 eligible children,
with a 44% participation rate.
Table 1 describes the characteristics of the CBGS cohort. Demographic data were not
available for non-participating women. However, the recruited term and late preterm
(gestational age > = 36 weeks) mother-infant dyads were similar to the overall population
of births at the Rosie Maternity Hospital, as shown in Table 2, albeit CBGS mothers
were slightly older and more likely to be primiparous.
Table 1.
Cohort characteristics (N indicates the total sample with available data)
Continuous variables
N
Mean (SD)
Categorical variables
N
% (with outcome specified)
Parental
Ethnicity
1619
Maternal age (years)
1338
33.5 (4.3)
White Caucasian
95.3
Paternal age (years)
1291
35.7 (5.4)
Black
1.3
Maternal height (cm)
1237
165.9 (7.2)
Asian
1.7
Maternal pre-pregnancy weight (kg)
1203
66.3 (12.4)
Other
1.7
Maternal pre-pregnancy BMI (kg/m2)
1173
24.1 (4.6)
Maternal pregnancy weight gain (kg)
866
8.2 (6.7)
Maternal education
780
Index of multiple deprivation
920
8.9 (4.2)
University degree-level
62.7
Other
37.3
Smoking during pregnancy
1575
5.5
Maternal weight
1173
Overweight (BMI > 25)
30.6
Obese (BMI > 30)
9.9
Primiparous pregnancy
1629
43.4
Recorded diabetic pregnancy
1562
7
Birth
Gestational age
1638
39.8 (1.6)
Birthweight (kg)
1657
3.47 (0.55)
Small for gestational age
1657
1.1
Newborn length (cm)
1599
51.4 (2.6)
Large for gestational age
1657
2.7
Newborn head circumference (cm)
1602
35.3 (1.7)
Newborn mean skinfold thickness (mm)
1599
6.3 (1.6)
Delivery (caesarian delivery)
1630
30.2
Infant sex (male)
1660
51.7
Infancy
3-month weight (kg)
1340
6.15 (0.83)
Rapid infancy weight gain 0–12 months
1179
3-month length (cm)
1335
61.1 (2.6)
Catch-up weight gain
26
3-month head circumference (cm)
1342
40.7 (1.4)
Normal weight gain
46.7
3-month mean skinfold thickness (mm)
1342
10.8 (2.0)
Catch-down weight gain
27.3
12-month weight (kg)
1182
9.97 (1.19)
12-month length (cm)
1175
75.9 (2.9)
12-month head circumference (cm)
1179
46.4 (1.4)
12-month mean skinfold thickness (mm)
1178
11.3 (2.1)
18-month weight (kg)
1094
11.36 (1.32)
18-month length (cm)
1096
82.2 (3.2)
18-month head circumference (cm)
1088
47.8 (1.5)
18-month mean skinfold thickness (mm)
1096
10.8 (2.1)
24-month weight (kg)
1051
12.61 (1.47)
24-month length (cm)
1043
87.8 (3.4)
24-month head circumference (cm)
1051
48.7 (1.5)
24-month mean skinfold thickness (mm)
1057
10.3 (2.2)
Infancy nutrition
Age stopped breastfeeding (months)
436
5.3 (3.1)
3-month feeding
1319
Age started solids (months)
652
5.0 (0.9)
Exclusively breastfed
42.2
Mixed-fed
24.9
Exclusively formula-fed
29
S'tarted solids
3.9
Table 2.
The Cambridge Baby Growth Study cohort compared with all infants born at term (gestational
age ≥ 36 weeks) at the Rosie Maternity Hospital in 2005 (midway through the study).Mean
(SD) or % are shown
CBGS
Rosie Maternity Hospital
N = 1585
N = 4670
Birthweight (kg)
3.52 (0.49)
3.47 (0.49)
Gestational age (weeks)
40.0 (1.3)
40.0 (1.3)
Maternal age
33.5 (4.3)
30.7 (5.6)
Caucasian ethnicity
95%
91%
Spontaneous vaginal delivery
60%
60%
Primiparous
43%
22%
Index of multiple deprivation
9.0
11.5 (Cambridgeshire)
How often have they been followed?
The study protocol included visits across antenatal, perinatal and postnatal periods,
with a wide range of data collection (Table 3).
Table 3.
Study schedule
Pregnancy (weeks)
Infancy (months)
Childhood (years)
Father
12
28
36
Birth
6 weeks
3
12
18
24
3–4
5–10
Physical measures
Anthropometry
•
•
•
•
•
•
Anogenital distance
•
•
•
•
•
Abdominal ultrasound
•
•
•
•
DXA scan
•
Pubertal assessment
•
Blood pressure
•
•
•
•
Biological Samples
Venous blood
•
OGTT
•
•
Cord blood and placenta
•
Dried blood spot
•
•
•
•
Breast milk
•
DNA sample
•
•
•
Questionnaires
Perinatal
•
Postnatal
•
Temperament
•
3-day food diary
•
•
Pre-school activities inventory
•
Pregnancy Visits
Antenatally, women were seen during each trimester of pregnancy. This included an
early pregnancy maternal venous blood sample at approximately12 weeks of gestation
for biochemical analytes and DNA extraction, a 75-g oral glucose tolerance test (OGTT)
at 28 weeks and administration of a questionnaire on parental demographics and environmental
exposures, including diet and chemical exposure, which was returned in late pregnancy.
Birth visit
Cord blood and placental samples were collected at birth where possible. Birthweight
was recorded at delivery by midwives and obtained from the hospital notes. Infants
were also examined as newborns, as early as possible in the first 2 weeks of life,
either in hospital or at home visits, with further anthropometric measurements as
detailed below. Infant DNA was extracted from the cord blood sample, where available.
Infancy visits
Infants were seen at research clinic visits at 3, 12, 18 and 24 months of age. These
visits included detailed longitudinal anthropometric measurements and questionnaires
on feeding, behaviour and illness. Biological samples consisted of repeated dried
blood spot collection, breast milk samples and infant venous or mouth swab samples
for DNA when cord blood was not available. A mouth swab was also obtained from the
father. Clinics were held in two locations due to the large geographical catchment
area of the Rosie Maternity Hospital: at the Cambridge Biomedical Campus (the NIHR-Wellcome
Trust Clinical Research Facility or the MRC Epidemiology Clinical Research Facility)
and at the Princess of Wales Hospital, Ely.
Mothers and infants who completed the initial follow-up to 2 years of age were very
similar to those lost to follow-up, although they were slightly older and more likely
to be of Caucasian ethnicity (Table 4).
Table 4.
Mother-infant dyads who completed infancy assessments to age 2 years (N = 1061) compared
with those infants lost to follow-up (N = 597)
Followed-up
Lost to follow-up
Mean (SD)
Mean (SD)
Birthweight (kg)
3.50 (0.54)
3.44 (0.55)
Gestational age (weeks)
39.8 (1.6)
39.7 (1.7)
Maternal age (years)
33.7 (4.3)
32.8 (4.6)
Caucasian ethnicity
97%
92%
Spontaneous vaginal delivery
58%
60%
Primiparous
43%
43%
Index of multiple deprivation
8.9 (3.6)
9.0 (2.2)
Exclusively breastfed at 3 months
43%
40%
Childhood visit
A Pre-school Activities Inventory (PSAI) questionnaire was posted to a subgroup of
194 mothers of 3–4-year-old children in 2010–11 (with 80% response rate). Children
aged 5–10 years are currently being re-recruited into a follow-up phase.
Throughout the study period, families have been kept informed of study updates and
activities, with biannual posted or e-mailed newsletters, and through two study events
for participating families in March 2010 and May 2011.
What has been measured?
Anthropometry
A team of four trained paediatric research nurses carried out all the measurements.
Infancy anthropometry included weight and length, and skinfold thickness measurements
at four sites (triceps, subscapular, flank, quadriceps). In September 2006, abdominal
ultrasound assessment of visceral and subcutaneous-abdominal fat depths was introduced
into the protocol (
N
= 498 at 3 months;
N
= 582 at 12 months,
N
= 550 at 24 months), as well as waist circumference measurement. This novel ultrasound
method was validated in infants against magnetic resonance imaging (MRI).
4
All anthropometric measures are currently being repeated at the childhood follow-up
visit, with the addition of dual-energy X-ray absorptiometry (DXA) scans.
Anogenital distance (AGD) was measured using a modified version of a previously published
method,
5
defined as the distance from the centre of the anus to:the junction of smooth perineal
skin and rugated skin of the scrotum in boys; and the lower vaginal opening in girls,
as well as penile length in boys, using Vernier calipers.Measurement of AGD was added
to the CBGS protocol in 2006.
For all physical measurements, there were regular training and quality control sessions
and the technical error of measurements (TEM) within and between observors was calculated.
Over the study period, the relative intraobserver TEM ranged as follows: for infant
length, between 0.03 and 0.50%; for individual skinfold thickness measurements, between
0.4 and 2.8%; for intra-abdominal depth, between 0.3 and 1.7%; and for subcutaneous
abdominal fat depth, between 1.1 and 2.6%. Relative interobserver TEM were: for infant
length, 0.7%; for individual skinfold thickness measurements, 2.0 to 3.2%; for intra-abdominal
depth, 3.2%; and for subscutaneous abdominal fat depth, 3.6%. Interobserver TEM for
AGD was 9.6% for males and 5.7% for females.
Biological specimens
During pregnancy, a non-fasting venous blood sample was collected from mothers at
approximately 12 weeks of gestation, into endocrine disrupting chemicals (EDC) contamination-free
glass containers. EDCs, including phthalate metabolites and phenols of mothers with
singleton male pregnancies have been measured using liquid chromatography tandem mass
spectrometry. A fasting 75 g-OGTT was completed at 28 weeks of gestation in 1071 women.
Cord blood and placental tissue were collected when possible. RNA was extracted from
placental tissues (N = 420). Genomic DNA was extracted from cord blood samples, maternal
and/or infancy blood samples or mouth swabs, and paternal mouth swabs for single nucleotide
polymorphism(SNP) genotyping. A total of 845 trios have been extracted and used in
analyses.
Dried blood spot samples (DBS) were collected repeatedly between 3 to 24 months of
age. Capillary heel-prick blood sampling was used, and single drops of blood were
added to untreated filter paper (Ahlstrom 226, ID Biological Systems), and stored
at −20°C. For analysis, 3.2-mm single spots were punched from the larger DBS. Since
DBS have not been widely used, work was done to investigate effects of storage conditions
and time,
6
,
7
and to validate assays converted from plasma assays—for example IGF-1, other hormones,
metabolites and lipidomic profiles.
7
,
8
Mothers who breastfed their infants were asked to hand-express a pooled hindmilk samplebetween
4 and 8 weeks postnatally. After feeding their infant, theyexpressed from the same
breast as they had last fed their infant from, and repeated this process several times,
so that a total of 100 mls of hindmilk was collected over a 2-week period, in order
to reduce within-day and day-to-day variations. A total of 641 pooled breast milk
samples were collected and stored at −20°C, until processed at a single time point.
Questionnaires
Several questionnaires were administered. The perinatal questionnaire, completed in
late pregnancy, included questions on socio-demographic status, self-reported maternal
(pre-pregnancy) and paternal anthropometry, medications,environmental exposures and
dietary and medical information. At 3 months of infancy age, the postnatal questionnaire
included questions on infant feeding practice (exclusive breastfeeding, mixed feeding
or exclusive formula feeding), health and illnesses. The revised infant behaviour
questionnaire was used to assess three main dimensions of infant temperament.
9
At the 12-month and 24-month clinic visits, a 3-day diet diary was collected. Dietary
data, measured using household measures (teaspoon, cup etc.), age-appropriate portion
sizes and home-made food recipes, were then coded by trained dietary assessment assistants
using the Human Nutrition Research (HNR) in-house programme DINO (Diet in Nutrients
Out), based on McCance and Widdowson The Composition of Foods, 6th edition.
10
The DINO output generated a food code and weight for each food item recorded. From
this information, total energy intake was calculated, as well as dietary intake of
fat, saturated fat, protein, carbohydrate and specific micronutrients.
What has been found? Key findings and publications
Findings from CBGS to date have mainly focused on the antenatal and early postnatal
determinants of infancy body size, male genital development and exposures influencing
maternal metabolism. We have shown that DBS are a useful biological medium and many
serum assays can be adapted, allowing measurement of early -ife circulating hormones,
metabolites and lipids. Our findings have therefore expanded current knowledge around
infancy growth and physiology, related to both prenatal and early postnatal exposures,
in particular those of nutrition. Work with DBS assays has begun to fill a gap in
the literature with respect to infancy hormones and metabolites, from studies on cord
blood to those in childhood. Additionally, further knowledge of maternal and infancy
metabolism may allow biomarkers for fetal and infancy growth. Our main findings from
the cohort are summarized below.
Antenatal determinants of infancy size and maternal metabolism
It is clear that antenatal determinants of infant size include environmental in utero
exposures and genetic influences, but also their interactions. We have shown that
maternal fasting glucose levels and maternal pre-pregnancy BMI are both independently
positively related to offspring adiposity in the non-diabetic population.
11
Whereasaternal glycaemia was correlated with infant adiposity at birth, but not afterwards,
maternal pre-pregnancy BMI showed increasingly strong associations with offspring
adiposity as infants aged.
Fetal imprinted genes
We demonstrated that fetal SNP genotypes at the paternally-imprinted (maternally-expressed)
gene H19 were associated with offspring size at birth.
12
Fetal genotypes at paternally-expressed imprinted genes were also found to influence
birthweight, likely indirectly by affecting maternal metabolism. Fetal genotypes at
the paternally-expressed gene IGF2 (encoding insulin-like growth factor-2) were associated
with maternal glucose levels during pregnancy,
13
and preliminary work in the CBGS has also suggested that polymorphic variation in
various paternally-expressed fetal imprinted genes may also affect maternal blood
pressure.
14
Infancy nutrition
Metabolic markers of infancy nutrition
Three-month insulin-like growth factor 1 (IGF-1) levels were analysed in 566 DBS samples
and found to be higher in formula-fed infants than in breastfed infants.
8
Extensive work using high-resolution mass spectrometry has also demonstrated striking
differences between the DBS lipidomic profiles of breastfed and formula-fed infants
at 3 months of age (N = 241); differences were evident across all main lipid classes,
with mixed-fed infants showing intermediate profiles.
15
Breastfed infants had higher levels of cholesterol esters, lower total phosphatidylcholines
(PC) and specific differences in many individual PCs and sphingomyelin lipid species.
Age of weaning
Age of weaning (introduction of semi-solid or solid foods) was inversely related to
infancy weight and length, but these associations are likely to be due to reverse
causality.
16
Consistent with evidence synthesized across other studies, these findings suggest
that infants who grow faster are more likely to be weaned earlier than other infants,
but that in turn age at weaning has neutral effects on infant growth.
Feeding practice and infant behaviour
Maternally-rated infancy temperament differed between breast/mixed-fed and formula-fed
infants at 3 months of age.
17
Breastfed and mixed-fed infants were rated as having more challenging temperaments,
with lower soothability, greater distress and less positive response to stimulation.
These results may allow better understanding and expectation of normal infant temperament
and this knowledge, with appropriate support, could be used to promote successful
breastfeeding.
Regulation of postnatal growth
Hormones
We demonstrated that infant DBS IGF-1 levels at age 3 months predicted subsequent
faster linear growth and lesser gains in adiposity, up to the age of 12–18 months.
8
It is likely that other hormones mediate the faster gains in adiposity seen in contemporary
milk formula-fed infants, and we are currently investigating several hormones, including
leptin, which are positively associated with weight and adiposity in infancy. Preliminary
work has also suggested that several specific lipid species in the infancy lipidome
are related to 3- and 12-month weight.
15
Genes
An individual-level meta-analysis of CBGS and three other European birth cohorts showed
that adult obesity-risk alleles were not associated with birth size, but were positively
associated with body size at 1 year, with strengthening correlations seen across the
first 5 years of life.
18
These alleles were not only associated with fat mass, as seen in older populations,
but had a symmetrical influence on both body size and body composition (lean and fat
mass). Of note, these alleles showed less influence on infants born with low or high
birthweights than on those of average birthweight.
Epigenetics
DNA methylation patterns have been measured in selected sub-groups—infants of mothers
with gestational diabetes and infants demonstrating rapid postnatal weight gain over
the first year of life—and compared with a group of control infants. We found multiple
differentially methylated sites, including at or near genes with known relevance to
growth and diabetes, and several methylation differences that were common to infants
of mothers with gestational diabetes and infants who showed rapid infancy weight gain.
19
This work suggests that similar epigenetic changes may occur after different in utero
exposures, and may possibly contribute to the same long-term phenotype, for example
with increased risk of obesity and type 2 diabetes.
Male reproductive development
CBGS is one of the largest cohort studies with repeated measurements of infancy penile
length, testicular descent and anogenital distance (AGD) (a marker of androgen exposure).
The study has generated normative data for infancy anogenital distance, for both boys
and girls.
20
AGD increased from birth up to age 12 months in a sexually-dimorphic pattern, and
was positively correlated with penile length at birth. These normative data will be
useful as markers of androgen action in further studies investigating environmental
effects on genital development. CBGS data also showed a higher incidence of congenital
cryptorchidism than reported in earlier UK studies.
21
We are currently investigating a possible role for EDCs in male reproductive disorders,
exploring data on maternal first-trimester serum phthalate and phenol levels. We are
also exploring how EDCs affect maternal metabolism during pregnancy.
What are the main strengths and weaknesses?
Particular strengths of the CBGS cohort design include the rich collection of biological
data and samples, including on maternal metabolism during pregnancy, environmental
and dietary exposures and infant and breast milk samples. Information was obtained
on both parents, and involvement of fathers meant that it was possible to analyse
DNA trios, from mother, father and infant.
Multiple collaborations have enabled diverse laboratory analyses, including state-of-the-art
mass spectrometry for novel metabolite and lipid profiling in infancy. As reflected
by a dearth of reported data on hormone and metabolite levels between birth and mid-childhood,
a major strength of CBGS is the collection of biological samples (largely by DBS sampling)
from 3 months of age, as well as the large number of breast milk samples and DNA trios.
Limitations of CBGS include its single-site design and prolonged recruitment over
an 8-year period. From birth to age 2 years, there was a 34% attrition rate which
is comparable to other UK longitudinal cohorts. There are advantages and disadvantages
of its South Cambridgeshire setting. The cohort is almost exclusively White Caucasian,
which reduces confounding but also limits generalizability to other settings and populations.
Additionally, the cohort has high rates of exclusive breastfeeding, compared with
the whole of the UK. However, this improves the power for differences between breastfed
and formula-fed infants to be observed.
A limitation of the body composition assessments is the absence of lean mass estimation,
other than by extrapolation from skinfold thicknesses. With increasing evidence supporting
the importance of the first few months of life as a critical period for long-term
outcomes, rather than just the first year of life,
22–24
it would have been beneficial to include more frequent assessments, particularly in
very early infancy.
Can I get hold of the data? Where can I find out more?
Anonymized data are available to other investigators through collaborative agreements,
and the CBGS co-investigators welcome formal or informal proposals and will consider
these at their bimonthly meetings. Please contact Dr Carlo Acerini [cla22@cam.ac.uk].
CBGS in a nutshell
The CBGS is a prospective, observational pregnancy and birth cohort, primarily investigating
antenatal and postnatal determinants of infancy growth and development.
A total of 2229 pregnant women were recruited at approximately 12 weeks of gestation
from one tertiary hospital in Cambridge, UK; 1658 newborns entered the infancy follow-up,
born between 2001 and 2009.
Detailed information collected during pregnancy included a first-trimester blood sample,
oral glucose tolerance test and questionnaire data for demographics, medical history
and environmental and dietary exposures.
Infants were seen newborn and at 3, 12, 18 and 24 months of age, with longitudinal
anthropometric measures, dried blood spot samples and questionnaire data. Breast milk
and genetic data are also available.
Any researcher interested in collaborating with the CBGS should contact Dr Carlo Acerini
[cla22@cam.ac.uk].
Funding
The Cambridge Baby Growth Study has been supported by the European Union Framework
V, the World Cancer Research Foundation International, the Medical Research Council,
the NIHR Cambridge Comprehensive Biomedical Research Centre, the Newlife Foundation
for disabled children, the Mothercare Group Foundation, Mead Johnson Nutrition, the
Evelyn Trust, the Wellbeing of Women, Diabetes UK and a collaborative research grant
from the European Society for Paediatric Endocrinology.