A thinner endometrium is associated with lower newborn birth weight during in vitro fertilization–frozen-embryo transfer: a cohort study

Figures & data

Figure 1. The process for case screening.

Table 1. Characteristics of patients at baseline and characteristics of frozen embryo transfer treatment.

Characteristic	Endometrial thickness (mm)			Statistic	p
	<8	8–12	>12
Woman’s age (years)	31.9 ± 3.963	31.30 ± 3.990	31.54 ± 3.847	4.942	0.007^a
Body mass index	22.643 ± 3.0349	22.733 ± 3.2749	23.417 ± 3.3216	5.078	0.006^a
Infertility type
Primary	42.5% (215/506)	58.2% (1549/2661)	68.3% (155/227)	56.411	0^b
Secondary	57.5% (291/506)	41.8% (1112/2661)	31.7% (72/227)
Infertility factor
Pelvis and fallopian tube	43.0% (172/506)	33.5% (891/2661)	39.6% (90/227)	23.653	0.003^b
Ovary	9.5% (48/506)	8.4% (224/2661)	4.4% (10/227)
Male	10.7% (54/506)	17.0% (453/2661)	17.6% (40/227)
Multiple factors	41.7% (211/506)	36.9% (982/2661)	32.6% (74/227)
Unexplained	4.2% (21/506)	4.2% (111/2661)	5.7% (13/227)
Number of years of infertility	4.217 ± 2.7961	4.471 ± 2.7646	4.480 ± 2.8647	1.811	0.164^a
Endometrium-preparation protocol
Natural cycle	66.6% (337/506)	70.5% (1876/2661)	162/227 (71.4%)	3.3	0.192^b
Hormone-replacement cycle	33.4% (169/506)	29.5% (785/2661)	65/227 (28.6%)
Follicle-stimulating hormone	6.37758 ± 2.118024	6.33828 ± 2.167308	6.07969 ± 1.988762	1.647	0.193^a
Luteinizing hormone	4.89 ± 3.39	5.19 ± 20.21	4.39 ± 2.52	0.232	0.793^a
Prolactin	14.77 ± 8.78	15.42 ± 12.81	14.31 ± 7.41	1.282	0.277^a
Estradiol	45.44 ± 23.51	45.44 ± 23.51	44.11 ± 21.44	0.262	0.77^a
Progesterone	2.30 ± 1.39	2.56 ± 6.02	2.22 ± 1.08	0.765	0.465^a
Testosterone	43.50 ± 21.77	41.35 ± 19.46	39.87 ± 15.71	3.188	0.041^a
Number of embryos transferred	1.92 ± 0.474	1.89 ± 0.510	1.82 ± 0.459	2.87	0.057^a
Number of implanted embryos	1.17 ± 0.395	1.16 ± 0.382	1.15 ± 0.386	0.207	0.813^a

^a One-way ANOVA. Values are mean ± SD.

^b Pearson chi-square test. Values are frequency (percentage).

Table 2. Pregnancy complications and neonatal outcomes.

Outcome	Endometrial thickness (mm)			Statistic	p
	<8	8–12	>12
Number of days of pregnancy	270.04 ± 15.246	272.7 ± 12.875	272.31 ± 14.222	8.448	0^a
Prevalence of preterm birth	14.4% (73/506)	8.9% (236/2661)	8.8% (20/227)	15.22	0^b
Prevalence of very-preterm birth	2.4% (12/506)	1.2% (32/2661)	1.3% (3/227)	4.261	0.119^b
Newborn sex
Male	50.2% (254/506)	49.8% (1324/2661)	54.2% (123/227)	1.643	0.44^b
Female	49.8% (252/506)	50.2% (1337/2661)	45.8% (104/227)
Cesarean section	73.3% (371/506)	68.8% (1832/2661)	63.4% (144/227)	7.714	0.021^b
Newborn birth weight	3254.10 ± 609.96	3342.34 ± 529.198	3407.11 ± 532.618	7.871	0^a
Birth-weight classification
Prevalence of low birth weight	2.1% (9/432)	1.5% (36/2422)	0% (0/205)	4.836	0.304^b
Prevalence of normal birth weight	88.7% (838/432)	90.3% (2186/2422)	90.7% (186/205)
Prevalence of macrosomia	9.3% (40/432)	8.3% (200/2422)	9.3% (19/205)
Gestational diabetes mellitus	6.1% (31/506)	8.2% (218/2661)	12.8% (29/227)	9.212	0.01^b
Hypertension in pregnancy	5.3% (27/506)	4.5% (120/2661)	7.0% (16/227)	3.319	0.19^b
Placenta previa	0.6% (3/506)	0.8% (21/2661)	0% (0/227)	1.253	0.586^c
Premature rupture of membranes	3.6% (18/506)	1.9% (51/2661)	3.1% (7/227)	6.021	0.049^b

^a One-way ANOVA. Values are mean ± SD.

^b Pearson chi-square test. Values are frequency (percentage).

^c Fisher’s exact test. Values are frequency (percentage).

Table 3. Univariate linear regression for newborn birth weight.

Factor	β	p
Woman’s age	−1.471	0.53
Body mass index	15.185	0
Infertility type	−32.85	0.081
Ovary factors (dummy variable)	−43.438	0.229
Male factors (dummy variable)	0.436	0.988
Multiple factors (dummy variable)	−8.791	0.691
Unexplained infertility (dummy variable)	−2.59	0.957
Number of years of infertility	3.76	0.263
Endometrium-preparation protocol	31.946	0.116
Follicle-stimulating hormone	−9.914	0.024
Luteinizing hormone	0.855	0.105
Prolactin	0.154	0.849
Estradiol	−0.974	0.013
Progesterone	1.152	0.521
Testosterone	−0.004	0.993
Number of embryos transferred	−27.882	0.133
Number of implanted embryos	−98.77	0
Number of days of pregnancy	23.976	0
Newborn sex	−97.784	0
Delivery mode	128.83	0
Gestational diabetes mellitus	6.652	0.845
Hypertension in pregnancy	325.019	0
Placental position	538.572	0
Premature rupture of membranes	855.715	0
EMT 8–12 mm	88.238	0.001
EMT >12 mm	153.011	0

Dummy variables were used for the infertility factors, with pelvic tubal factors as the reference group. Dummy variables were used for the EMT groups, with EMT <8 mm as the reference group. EMT: endometrial thickness.

Table 4. Multivariate linear regression model for newborn birth weight.

Model	β	t	p
(Constant)	−4468.101	−17.741	0.000
EMT 8–12 mm	25.942	1.243	0.214
EMT >12 mm	85.107	2.494	0.013
Hypertension in pregnancy	123.483	3.523	0.000
Premature rupture of membranes	148.859	2.835	0.005
Placental position	182.342	2.018	0.044
Newborn sex	−126.242	−8.544	0.000
Number of days of pregnancy	23.837	39.586	0.000
Delivery mode	130.487	8.049	0.000
Number of implanted embryos	−55.023	−2.722	0.007
Follicle-stimulating hormone	−6.215	−1.779	0.075
Estradiol	−1.124	−3.659	0.000
Body mass index	22.218	9.299	0.000

Dummy variables were used for infertility factors, with pelvic tubal factors as the reference group. Dummy variables were used for EMT groups, with EMT <8 mm as the reference group. EMT: endometrial thickness.

Table 5. Model summary.

R	R^2	Adjusted R^2	Standard error	F	p value
0.637	0.406	0.404	418.018	183.355	<0.001

Independent variables: (constant), EMT group (<8 mm = 1, 8–12 mm = 2, and 12 mm = 3), E2, FSH, Number of implanted embryos, Placenta previa = 1, Normal placental position = 2, Male = 1, Female = 2, Premature rupture of membranes = 1, Non-premature rupture of membranes = 2, Hypertension in pregnancy = 1, Normal blood pressure = 2, Vaginal delivery = 1, Cesarean section = 2, Days of pregnancy.

Data availability statement

All authors had full access to the data and materials. The data supporting the conclusions of this article are included within the article.