Abstract
Objective To explore whether thin endometrial thickness (EMT)
is associated with singleton low birth weight (LBW) from single fresh
blastocyst transfer.
Design Retrospective cohort study.
Setting Reproductive center.
Population All women were ≤40 years old and underwent single
fresh blastocyst transfer and singleton live birth.
Interventions None.
Methods Multivariable logistic regression was used to evaluate
the association between thin EMT and singleton LBW.
Main outcome measures LBW was the primary concern of this
study.
Results In total, 2847 women met the study inclusion criteria.
The neonatal birthweight in the EMT ≤7.5 mm group was significantly
lower than that in the EMT 7.6~12.0 mm and EMT
>12.0 mm group (P<0.001). The rate of LBW in the
EMT ≤7.5 mm group was 24.9%, which was significantly higher than the
4.0% in the EMT 7.6~12.0 mm group and the 5.3% in the
EMT >12.0 mm group (P<0.001). The total neonatal
malformation rate was similar between the groups (1.1%, 0.8% and
1.5%, P=0.21). After multiple logistic regression analysis, EMT≤7.5 mm
was an independent risk factor for LBW (adjusted odds ratio [AOR]:
4.39, 95% CI: 1.85~10.46, P<0.001).
Conclusions Thin EMT (≤7.5 mm) on the hCG trigger day is an
independent risk factor for singleton LBW from single fresh blastocyst
transfer. The neonatal birthweight in the EMT ≤7.5 mm group was
significantly lower than that in the EMT 7.6~12.0 mm and
EMT >12.0 mm groups. The total neonatal malformation rate
was comparable between the groups.
Keywords low birth weight, endometrial thickness, blastocyst,
birthweight
Tweetable abstract Thin endometrium thickness is an independent
risk factor for singleton LBW from single fresh blastocyst transfer.
Introduction
Infertility affects approximately 10% of reproductive-aged couples
worldwide1. In vitro
fertilization (IVF) is widely used and has become the most effective
treatment for infertility caused by tubal factors or other factors as
well as unexplained
infertility2. The safety
of IVF offspring has always been our focus. However, adverse pregnancy
and perinatal outcomes, such as low birth weight (LBW), preterm birth,
pregnancy-induced hypertension and gestational diabetes, are increased,
even for singleton
births3-5. However, the
exact biological mechanism leading to adverse perinatal outcomes is
unclear. Some studies have shown that infertility disease itself is the
main reason for the poor perinatal outcome of assisted reproductive
technology (ART) singleton
offspring6. However, an
increasing number of studies have shown that the process of ART,
including exposure to superphysiological doses of hormones and embryo
manipulation in the laboratory, may have an adverse effect on the
perinatal outcome7-10.
Furthermore, defective placentation, particularly in patients with thin
endometrium thickness (EMT), may be an important cause of these poor
perinatal outcomes11,
12. However, there is no exact
definition of “thin EMT”, and most studies have reported cutoff values
of 7 mm, 7.5 mm or 8
mm13-17. Thin EMT is an
independent factor affecting the ART pregnancy
rate18. In recent
years, studies have shown that a thin EMT is not only related to the
pregnancy rate but also to perinatal complications and offspring
safety13-15. However,
there are few related studies and limited data, and cleavage-stage
embryos and blastocysts have not been studied and analyzed separately.
Different stages of embryo development may have different effects on
offspring safety19,
20. Therefore, in this study, we
selected patients with single fresh blastocyst transfer and analyzed the
effect of thin EMT on the LBW of singleton offspring.
Materials and methods
Study design and population
A retrospective cohort study was approved by the review board of The
Third Affiliated Hospital of Zhengzhou University. All patients who
initiated their first in vitro fertilization (IVF)/intracytoplasmic
sperm injection (ICSI) cycles at the Reproductive Center of Third
Affiliated Hospital of Zhengzhou University between January 2016 and
February 2020 were analyzed for potential inclusion. Inclusion criteria
for women aged ≤40 years, undergoing single fresh blastocyst transfer
(D5/D6) and having a singleton live birth. Cycles with oocyte donation,
vanishing twins, adenomyosis, uterine malformations, endometrial polyps,
preimplantation genetic testing (PGT) or incomplete records were
excluded.
Clinical and laboratory protocols
All of the women were undergoing standardized GnRH agonist (GnRH-a) or
flexible GnRH antagonist (GnRH-anti) protocols. The details of
controlled ovarian hyperstimulation (COH) were described in our previous
study3. In brief, for
the GnRH-a protocol, protracted downregulation with GnRH-a (Diphereline,
lpsen, France) 3.75 mg was performed on the second or third day of the
menstrual cycle, followed by daily use of recombinant
follicle-stimulating hormone (rFSH), which was based on the ovarian
response. In the flexible GnRH-anti group, rFSH was initiated on the
second or third day of the menstrual cycle. Injection of GnRH-anti at
0.25 mg/day commenced once the diameter of the dominant follicle reached
14 mm and was continued up to the trigger day.
For both protocols, the dose of rFSH was adjusted according to the
follicle response. As soon as the diameter of the dominant follicle was
greater than 20 mm or when at least three follicles reached 18 mm,
ovulation induction was triggered with 5000 to 10000 IU hCG (Lizhu
Pharmaceutical Trading, China). Oocyte retrieval was performed 36 hours
later. Based on the sperm quality, conventional IVF or ICSI was
performed approximately 4 to 6 hours after follicular aspiration. In our
study, all women underwent single fresh blastocyst transfer on the fifth
day after fertilization. Routine corpus luteum support was initially
provided on the day of oocyte retrieval, namely, oral dydrogesterone (10
mg twice daily) (Abbott Co. America), and intravaginal administration of
90 mg of a progesterone sustained-release vaginal gel (Merck Co.
Germany) was given. Corpus luteum support was performed at least until
55 days after transplantation if pregnancy occurred.
EMT assessment and grouping
For EMT assessment, only three doctors with extensive experience in
transvaginal ultrasound monitoring performed the EMT assessment in our
reproductive center. EMT was measured in the sagittal plane. The
distance between the hyperechogenic interfaces between the endometrium
and the myometrium was recorded approximately 1 centimeter beneath the
uterine fundus. The EMT was recorded and analyzed in millimeters. The
EMT was taken from the hCG trigger day. Patients were categorized into
three groups depending on their EMTs: ≤7.5 mm, 7.6~12.0
mm and >12.0 mm. These cutoffs were selected as being close
to those in previous
studies13-15.
Outcome measures and definition
LBW was defined as a neonatal birth weight less than 2500
g21 and was the primary
concern of this study. The secondary outcome measures were neonatal
birthweight and neonatal congenital malformations, including trisomy
13/18/21, congenital heart disease, polydactyly/syndactyly and other
disorders.
Statistical analysis
All of the data analysis in our study were obtained via review of our
reproductive center’s medical records.
The one-sample Kolmogorov–Smirnov test was used to check for normality
of continuous variables. The Wilcoxon rank-sum test was used to assess
between-group differences in continuous variables with abnormal
distributions, and these variables are expressed as the median (P25,
P75). Categorical variables are presented as the number of cases (n) and
the percentage (%). The means obtained from chi-square analyses were
used to assess the differences between groups with Fisher’s exact test
when necessary. For LBW, multiple logistic regression was used to adjust
for the baseline characteristics between groups. Adjusted odds ratios
(AORs) with 95% confidence intervals (CIs) were calculated. All
statistical management and analyses were performed using SPSS software,
version 24.0. Statistical significance was set at p<0.05.
Results
Study population
In total, 2847 women who underwent single fresh blastocyst transfer and
delivered singleton births met the study inclusion criteria, including
181 women with EMT ≤7.5 mm, 1714 women with EMT 7.6~12.0
mm and 952 women with EMT >12.0 mm.
Characteristics of the study groups
Table 1 shows the demographic and clinical characteristics among the
three groups. There were no significant between-group differences in
maternal age, body mass index (BMI), duration of infertility, type of
infertility, infertility diagnosis, basal serum FSH level, number of
antral follicle counts, fertilization method, COH protocols, duration of
ovarian stimulation, dosage of gonadotropins, serum estradiol on the
trigger day or number of oocytes retrieved.
Neonatal outcomes
The neonatal birthweight in the EMT ≤7.5 mm group was significantly
lower than that in the EMT 7.6~12.0 mm and EMT
>12.0 mm group (EMT ≤7.5 mm, 3000 (2525,3350); EMT
7.6~12.0 mm, 3350 (3050,3650); EMT >12.0
mm, 3400 (3100,3690), P<0.001). The sex of the newborns was
comparable between groups (P=0.06). The rate of LBW in the EMT ≤7.5 mm
group was 24.9%, which was significantly higher than the 4.0% in the
EMT 7.6~12.0 mm and 5.3% in the EMT >12.0
mm group (P<0.001). The total neonatal malformation rate was
similar between the groups (1.1%, 0.8% and 1.5%, P=0.21).
Regarding the main outcome measures, LBW, to adjust for the influence of
confounding factors, we conducted a multiple logistic regression
analysis. The regression model included the following factors: maternal
age (continuous variable), BMI (continuous variable), type of
infertility (primary/secondary infertility), infertility diagnosis
(tubal/male/others), COH protocols (GnRH-a protocol/GnRH-anti protocol),
serum estradiol on the trigger day (<4000 pg/ml≥4000 pg/ml),
EMT (≤7.5/7.6~12.0/>12.0 mm), number of
gestational weeks (continuous variable) and sex of the newborn
(male/female). Compared with EMT 7.6~12.0 mm, EMT≤7.5 mm
was an independent risk factor for LBW (AOR: 4.39, 95% CI: 1.85~10.46,
P<0.001). In addition, BMI (AOR: 0.91, 95% CI: 0.84~0.99,
P=0.03), serum estradiol on the trigger day (AOR: 1.87, 95% CI:
1.16~2.21, P=0.03), gestational weeks (AOR: 0.37, 95% CI: 0.32~0.42,
P<0.001) and sex of the newborn (AOR: 1.99, 95% CI:
1.19~3.33, P=0.01) were risk factors for LBW. The detailed data are
described in Table 3.
Discussion
Main findings
In this single-center retrospective cohort study, a thin EMT (≤7.5 mm)
on the hCG trigger day was an independent risk factor for LBW of a
singleton live birth from a single fresh blastocyst transfer. The
neonatal birthweight in the EMT ≤7.5 mm group was significantly lower
than that in the EMT 7.6~12.0 mm and EMT
>12.0 mm groups. The total neonatal malformation rate was
comparable between groups.
Strengths and limitations
The first study to explore the relationship between thin EMT and
perinatal outcome was performed by Chung et
al.22. This study found
that EMT has a significant protective effect on perinatal outcomes,
namely, LBW and preterm birth; in other words, suboptimal endometrial
development is associated with adverse outcomes in pregnancies achieved
through IVF22. Another
early study explored the relationship between thin EMT and perinatal
outcome regarding the risk of placenta previa. Rombauts et al. found
that the risk of placenta previa was significantly higher in women with
EMT>12 mm than in women with an endometrial thickness of
<9 mm23. A
large retrospective cohort study including 6181 singleton newborns found
that a thin EMT was associated with a lower mean birthweight and
birthweight Z-score15.
Recently, Guo et al.13reported that the risk of being small for gestational age (SGA) was
increased approximately twofold in women with EMT≤7.5 mm compared with
women with EMT >12 mm after fresh embryo transfer. However,
to the best of our knowledge, the current study did not analyze the
effect of EMT on the hCG trigger day on the neonatal birthweight from
single fresh blastocyst transfer. We know that the different stages of
embryo transfer, that is, the cleavage-stage embryo or the blastocyst,
may have different effects on perinatal
outcomes19,
24, 25.
However, single blastocyst transfer, compared with cleavage-stage embryo
transfer, can increase the pregnancy rate, and has a lower risk of
multiple births26;
moreover, single blastocyst transfer is increasingly widely used in
clinical practice and may become the recommended transplantation
strategy. Therefore, to reduce the influence of confounding factors, it
is necessary to further analyze the impact of EMT of single blastocyst
transfer on the safety of the offspring. In this study, thin EMT (≤7.5
mm) was an independent risk factor for LBW of a singleton live birth
from a single fresh blastocyst transfer.
In this study, to minimize the influence of confounding factors, only
single fresh blastocyst transfer was included. After analyzing and
adjusting for confounding factors by EMT grouping, this study showed
that thin EMT (≤7.5 mm) is an independent risk factor for LBW and
further confirmed the influence of EMT on neonatal birthweight, which is
an important supplement to current clinical research. To the best of our
knowledge, this is the first study to explore the effect of thin EMT on
singleton LBW from single blastocyst transfer. Only single-center data
were included to minimize the influence of confounding factors caused by
different EMT measurements, clinical protocols, and laboratory
operations. However, this investigation also has certain limitations. It
was a retrospective cohort study and did not further explore the
relevant biological mechanism by which thin EMT affects the incidence of
LBW.
Interpretation
The specific biological mechanism of the impact of thin EMT on newborn
weight and perinatal outcomes is still unclear and likely complex.
Research on the main mechanism focuses on the oxygen concentration.
After ovulation, the uterine spiral artery contracts, and the blood flow
on the surface of the endometrium decreases, thereby reducing the oxygen
concentration of the functional epithelium during embryo
implantation27. In
early pregnancy, hypoxic tension is one of the main conditions for
normal embryo implantation and fetal
development28. However,
the thinning or lack of a functional layer may cause more blood vessel
growth and higher oxygen concentrations in the basal endometrium of the
embryo. The subsequent high oxygen tension may affect embryo
implantation and placental development, thereby affecting the growth and
development of the
fetus29,
30. Another mechanism is related to
spiral arterial revascularization. In early pregnancy, spiral arterial
vascular remodeling is an important factor in determining placental
blood perfusion, and the lack of vascular remodeling will affect the
blood supply of the placenta and eventually lead to perinatal
complications, such as fetal growth restriction and pregnancy-related
hypertension31,
32. The uterine artery blood flow
resistance of a thin endometrium is high, and there is vascular
dysplasia. These changes may affect spiral artery vascular remodeling,
thereby affecting the development of the placenta and causing poor
perinatal outcomes12,
15.
Conclusion
In conclusion, this study showed that a thin EMT (≤7.5 mm) was
significantly correlated with neonatal birthweight and was an
independent risk factor for singleton LBW from fresh single blastocyst
transfer. In addition, the neonatal birth weight in the EMT ≤7.5 mm
group was significantly lower than that in the EMT
7.6~12.0 mm and EMT >12.0 mm groups. This
finding may be related to spiral arterial vascular remodeling and
placental development, and further basic research is needed to explore
its related biological mechanism. Therefore, for patients with thin EMT,
the perinatal period may require more attention from obstetricians and
pediatricians.
Disclosure of interests
All authors have nothing to disclose.
Contribution to authorship
WXL designed the study. DMZ and ZJW were involved in the data extraction
and analysis. LMM reviewed the data. DMZ and ZJW was involved in
drafting this article. All authors have approved the final version of
the manuscript.
Details of ethics approval
This study was approved by the ethics committee of The Third Affiliated
Hospital of Zhengzhou University (2020–107).
Funding
We did not receive any funding for this study.
Acknowledgments
We acknowledge the patients who participated in the study. We also thank
American Journal Experts for their professional manuscript-editing
service.