Factors associated with SARS-CoV-2 transplacental transmission.

Objective

Transplacental transmission of SARS-CoV-2 is a rare event, although severe cases have been described. We know that the transmission may occur through Hofbauer cells in a minority of cases. Therefore, other factors, such as placental expression of viral receptors, viral load, degree of inflammation, or some clinical features, might be involved in the transmission. We investigated these factors, and we hypothesized that these factors might play a relevant role.

Study Design

We observed a series of 6 cases of SARS-CoV-2 transplacental transmissions; as we suspected that the first ascertained case of transplacental transmission was linked to fetal distress, all these cases received fetal monitoring. These 6 cases presented placental positive real-time polymerase chain reaction (RT-PCR). Moreover, we recruited 4 other women affected by COVID-19 during the third trimester of pregnancy with positive placental RT-PCR but without transplacental transmission: these 6 and 4 cases constitute the group of 10 pregnancies complicated by COVID-19 and placental infection (C+P+; ie, the 6 transplacental transmissions were in this group). In the same period, we also recruited 10 women with COVID-19 during the third trimester of pregnancy and negative placental RT-PCR (C+P−; ie, pregnancies complicated by COVID-19 but without placental infection) and 11 healthy pregnant women without any SARS-CoV-2 infection (controls). Clinical management of all studied patients is described in the Supplemental Methods.

We performed a translational cohort study analyzing biological data (enzyme-linked immunosorbent assay [ELISA] for viral receptors, RT-PCR with viral load estimation, and gene sequencing, histology, and immunohistochemistry; details are available in the Supplemental Methods) in placentas obtained from the 3 groups. Further to the biological analyses, we compared clinical data and perinatal outcomes of pregnancies complicated by COVID-19 with transplacental transmission (n=6; ie, the cases of transplacental transmission included in the C+P+ group) vs those with COVID-19 but without transplacental transmission (n=14; ie, the cases [extracted from C+P+ and C+P− groups] of COVID-19 during pregnancy but lacking transplacental transmission; ie, all patients from our dataset not experiencing a transplacental transmission).

Results

Patients in the C+P− and C+P+ groups had mild or moderate COVID-19. Except for 2 cases (1 neonatal cerebral vasculitis and 1 fetal demise), the clinical history of the 6 transplacental transmissions was typical for their age (Supplemental Results). The 6 cases were classified as in utero confirmed (n=4) or in utero possible (n=2) transplacental transmission following the World Health Organization criteria, whereas they were classified as definite placental infections following the Eunice Kennedy Shriver National Institute of Child Health and Human Development criteria. Of note, 3 cases were infected with the wild-type virus, and 3 cases were infected with the alpha variant. The Table shows that pregnancies with and without transplacental transmission had similar basic data; nonetheless, surviving neonates more often had fetal distress needing cesarean delivery and neonatal intensive care unit (NICU) admission compared with controls. Viral load and expression of viral receptors in pregnancies were similar in all women (Supplemental Results). Pathology showed (1) massive fibrin perivillous deposition in >50% or <40% of the tissue in cases with or without transplacental transmission, respectively; (2) evidence or absence of diffuse intervillositis with fibrin deposition and necrosis in cases with or without transplacental transmission, respectively; and (3) diffuse or focal (at the villi surface) positive SARS-CoV-2 nucleoprotein immunohistochemistry in cases with or without transplacental transmission, respectively (Supplemental Results).

Table

Basic clinical data and perinatal outcomes of pregnancies complicated by COVID-19 during the third trimester of pregnancy with or without SARS-CoV-2 transplacental transmission

Basic data	Transmitted (n=6)	Nontransmitted (n=14)	P value
Maternal age (y)	28.1 (3.7)	30.4 (3.5)	.406
Gestational diabetes mellitus	1 (20.0)	3 (21.4)	1
Preeclampsia	0 (0)	0 (0)	NA
Small-for-gestational-age neonates	0 (0)	2 (14.3)	1
Exposure time (d)	0.7 (0.2–9.5)	1.3 (0.8–9.9)	.383
Cesarean delivery	5 (100.0)	5 (35.7)	.03a
Body mass index	25.7 (5.3)	26 (5.5)	.943
Parity	0 (0.0–2.5)	0 (0–0)	.364
Lymphocytopenia	3 (50.0)	6 (42.9)	1
High inflammatory reaction	3 (50.0)	5 (35.7)	.642
Thrombocytopenia	2 (33.3)	2 (14.3)	.549
Transaminitis	3 (50.0)	4 (28.6)	.613
Gestational diabetes mellitus	3 (50.0)	3 (21.4)	.303
Perinatal outcomes	Transmitted and alive (n=5)	Nontransmitted (n=14)	P value
Gestational age at the delivery (wk)	33.6 (4.2)	32.7 (6.1)	.751
Newborn birthweight (g)	2233 (838)	1859 (1088)	.468
Fetal distress	5 (100.0)	2 (15.3)	.002a
Arterial cord pH	7.24 (7.20–7.26)	7.29 (7.26–7.34)	.038a
5-min Apgar score	7 (3–10)	10 (8–10)	.240
Need for neonatal intensive care unit admission	5 (100.0)	6 (42.8)	.008a

Basic clinical data are considered in 6 cases of transplacental transmission (ie, those included in the C+P+ group), whereas perinatal outcomes are considered only in 5 surviving neonates (because one of these pregnancies ended in fetal demise and was not considered for this analysis). These pregnancies with transplacental transmission were compared with all patients from our dataset not experiencing a transplacental transmission: in other words, the 6 transplacental transmissions were compared with 14 cases of COVID-19 during the third trimester of pregnancy without transplacental transmission (ie, all cases enrolled in the C+P− and C+P+ groups lacking transplacental transmission). More details are available in the Supplemental Methods and Supplemental Results. Data are presented mean (standard deviation, number (percentage), or median (interquartile range), unless otherwise specified. Moreover, the data were analyzed using the Mann-Whitney, chi-square, or Fisher test, as appropriate.

C, COVID-19; NA, not applicable; P, placenta.

Vivanti. Transplacental SARS-CoV-2 transmission, placental inflammation, and fetal distress. Am J Obstet Gynecol 2022.

Significant P value.

Conclusion

Our findings suggested that (1) viral load and expression of viral receptors are not linked to the transplacental transmission and that (2) placental inflammation with a peculiar signature is evident in cases of transplacental transmission, which is associated with fetal distress, lower cord pH, and NICU admission. Interestingly, all live-born transmissions occurred in the setting of nonreassuring fetal heart tracings and/or prematurity. Fetal monitoring during mild-moderate COVID-19 is not firmly recommended, but our findings have raised the hypothesis that transplacental transmission might occur more often than thought. Maternal infection in proximity to delivery may be a risk factor for the transmission: this seems mainly because of the inflammatory placental damage, which is associated with the immune response at the maternal-fetal interface and increasing cytokines in the fetal circulation. This is a situation similar to the so-called “cytokine storm” observed during severe SARS-CoV-2 pneumonia: this excessive local response might lead to placental insufficiency and transplacental transmission. The knowledge accumulated so far has been provided mostly by case series: our study was about the mechanisms of SARS-CoV-2 transplacental transmission. Study limitations included the lack of data on viral variants and the small sample size possibly introducing type 2 error and selection bias (although groups were comparable—shown in the Supplemental Results) and limiting the possibility to study exposure time (ie, time from maternal infection to delivery) and viral receptors expression across gestational age.

Supplemental Table 1

Synopsis of main clinical features for the 6 cases of SARS-CoV-2 transplacental transmission

No.	Maternal age (y)	COVID-19 severity	Prenatal steroids	Cesarean delivery	CTG type	GA at birth (wk)	Birthweight (g)	Sex	5-min Apgar score	NICU admission	Neonatal complications	ACE2 (ng/mg ×10,000)	TMPRSS2 (ng/mg ×10,000)
1	29	Moderate	Yes	Yes	II	33	2130	Female	5	Yes	RDS	8.2	0.47
2	30	Mild	Yes	Yes	II	31	1600	Male	10	Yes	RDS	7.4	0.3
3	32	Moderate	No	Yes	III	29	1220	Female	7	Yes	RDS	7.5	0.35
4	30	Mild	No	Yes	II	40	3640	Male	10	Yes	TTN	0.9	0.01
5	23	Mild	No	Yes	III	35	2540	Male	2	Yes	Cerebral vasculitis	2.8	0.28
6	27	Mild	No	NA	NA	32	2248	Male	NA	No	Fetal death	16.1	1.3

Viral receptors (ACE2 and TMPRSS2) are normalized to the total protein content (shown in detail in the Supplemental Methods section).

ACE2, angiotensin-converting enzyme 2; CTG, cardiotocography; GA, gestational age; NA, not available; NICU, neonatal intensive care unit; RDS, respiratory distress syndrome (ie, hyaline membrane disease because of primary surfactant deficiency); TMPRSS2, transmembrane serine protease 2; TTN, transient tachypnea of the neonate.

Vivanti. Transplacental SARS-CoV-2 transmission, placental inflammation, and fetal distress. Am J Obstet Gynecol 2022.

Supplemental Table 2

Basic clinical details of pregnancies enrolled in the 3 groups

Variable	C+P− (n=10)	C+P+ (n=10)	C+P− vs C+P+: P value	Controls (n=11)	3-group comparison: overall P value
Age (y)	30.7 (3.8)	28.8 (2.9)	.308	33 (5.3)	.181
BMI	26.2 (5.5)	24.8 (4.8)	.621	24.2 (4.2)	.641
Preeclampsia	0 (0)	0 (0)	NA	1 (9.0)	.470
Gestational diabetes mellitus	1 (10.0)	3 (30.0)	.582	1 (9.0)	.349
IUGR	1 (10.0)	1 (10.0)	1	0 (0)	.279
Cesarean delivery	3 (30.0)	7 (70.0)	.179	4 (36.4)	.152
Gestational age at the delivery (wk)	33 (5.9)	33.4 (5.8)	.930	38.2 (4.6)	.07
Prematurity	6 (60.0)	6 (60.0)	1	2 (18.2)	.082
Newborn birthweight (g)	2003 (1151)	2039 (905)	.908	2784 (782)	.132
5-min Apgar score	10 (3.05–10.0)	10 (7.0–10.0)	.240	10 (10.0–10.0)	.485

C+/P− and C+/P+ indicate women affected by COVID-19 in the first trimester of pregnancy with negative (n=10) or positive (n=11) RT-PCR in placental tissue samples, respectively. Controls indicated healthy pregnant women (unaffected by COVID-19). BMI and Apgar score are dimensionless variables. Data are expressed as mean (standard deviation), median (interquartile range), or number (percentage), unless otherwise specified. Dichotomous data were analyzed using the chi-square or Fisher test, as appropriate. Continuous data were compared using the Student test (C+P− vs C+P+ comparisons) or with 1-way analysis of variance (3-group comparisons). P values are shown for the comparisons between C+P+ and C+P− groups and for the overall 3-group comparisons.

BMI, body mass index; C, COVID-19; IUGR, intrauterine growth restriction; NA, not available; P, placenta; RT-PCR, real-time polymerase chain reaction.

Vivanti. Transplacental SARS-CoV-2 transmission, placental inflammation, and fetal distress. Am J Obstet Gynecol 2022.

Supplemental Table 3

Estimated placental viral load and expression of viral receptors in women with COVID-19 during the third trimester of pregnancy with (belonging to C+P+ group) or without (belonging to the C+P− and C+P+ groups) SARS-CoV-2 transplacental transmission

Variable	Transmitted (n=6)	Nontransmitted (n=14)	P value
Ct value	16.2 (13.1–19.9)	0 (0.0–20.0)	.153
ACE2 (ng/μg×10,000)	7.4 (2.4–10.2)	8.4 (5.7–10.5)	.494
TMPRSS2 (ng/μg×10,000)	0.3 (0.2–0.7)	0.3 (0.2–0.9)	.659

Data are presented as median (interquartile range), unless otherwise specified. Data were analyzed using the Mann-Whitney test. The nontransmitted group included both infected and noninfected placentas. ACE2 receptor and TMPRSS2 were corrected for total protein content. Ct values are dimensionless numbers. All measurements were performed in duplicates.

ACE2, angiotensin-converting enzyme 2; C, COVID-19; Ct, real-time polymerase chain reaction cycles; P, placenta; TMPRSS2, transmembrane serine protease 2.

Vivanti. Transplacental SARS-CoV-2 transmission, placental inflammation, and fetal distress. Am J Obstet Gynecol 2022.