Progesterone is secreted from the corpus luteum in the luteal phase and prepares the endometrium for blastocyst implantation, reduces and inhibits maternal immune response against fetal semi-allograft, maintains decidualization [1, 2, 3]. Circulating levels of progesterone rise during pregnancy; the major origin is the corpus luteum until just about week 8 of pregnancy, and the placenta thereafter. One of the major mechanisms of progesterone action in maintaining pregnancy is inhibition of the contractions of the myometrium [4]. Sufficient oxygen and nutrient supply are another important factor that determines proper embryo development [5]. Efficient blood transport depends on an adaptive capacity of the uterine vascular system. Arterial blood is transported to the uterus by way of the uterine and ovarian arteries. The ascending branch of the uterine artery is an essential source of the blood supply to the embryo. The spiral arteries compose the terminal portions of the uterine vasculature and penetrate the implantation and placental site [6]. The spiral artery, which provides endometrial blood supply, undergoes remodeling in the early stages of pregnancy and thus uteroplacental resistance is significantly reduced. As a result, blood flow to the placenta increases significantly [7, 8]. These changes in the spiral artery can be evaluated by ultrasonography, and normal or abnormal findings can be recognized [9, 10]. Detection of the relationship between abnormal spiral artery findings and negative perinatal outcomes may create a chance for close follow-up and timely medical intervention for pregnancies at risk [11]. Various studies have been conducted to determine how valuable progesterone serum levels are in terms of predicting perinatal outcomes, and normal progesterone levels have been tried to be determined. As a result, it has been revealed that serum progesterone levels differ according to trimester and have wide ranges that cannot be clinically useful in predicting perinatal outcomes [12, 13]. Maternal factors that may affect serum progesterone levels such as body mass index (BMI), age, parity, smoking and ethnicity may also be effective in this [14]. It has been suggested that pregnancies with low serum progesterone levels and a high risk of miscarriage may benefit from progesterone therapy [15]. Progesterone can increase uteroplacental circulation by decreasing the pulsatility index and resistive index of spiral arteries with its effects on vascular structures. When Roberts et al. [16] detected maternal blood flow to the placenta at the 6th gestational week, they revealed that this flow did not increase significantly until the 13th week. As a result of the significant increase observed in the 13th week, changes were also detected in spiral artery Doppler findings. Changes in the spiral artery are extremely important for the course of pregnancy. As a result of a deficiency or disorder that may occur in this remodeling, high-speed maternal blood flow and oxidative load may occur to the placenta. The spiral arteries of the multiparous uterus are partially replaced by the connective tissue of the smooth muscle layer, with fragmentation and duplication of the elastica. Therefore, these persistent changes are thought to reduce uteroplacental resistance [17]. Mechanical-biochemical trophoblastic damage and increased apoptosis are observed in the placenta [18]. Placenta is a temporary organ connecting the fetus and her mother. The trophoblast is the main cellular component of the placenta. The normally functioning trophoblast is crucial for the establishment and maintenance of pregnancy, development of the placenta, remodeling of the spiral artery, and subsequently fetal growth and maturation [17]. Dysregulation of trophoblasts is associated with pregnancy complications including early pregnancy loss (EPL) and preeclampsia [18]. The function of trophoblasts is regulated by a group of genes, especially placenta-specific genes including placenta-specific protein 1 (PLAC1), syncytin and galectins. These genes regulate trophoblastic migration, invasion, proliferation, apoptosis, and metabolism [19].

There is no study in the literature that specifically examines the relationship between first trimester progesterone levels, endometrial flow and pregnancy outcomes. In this study, we aim to analyse the relationship between serum progesterone levels, endometrial flow and pregnancy outcome.

This observational retrospective study included singleton pregnant women aged 18–39 years, who underwent routine first-trimester prenatal examination (11–13 weeks + 6 days) between May 2019 and December 2019 at a tertiary hospital. In this study, historical records of progesterone levels, spiral artery Doppler findings and pregnancy outcomes of normal pregnant women who came to the clinic for the first trimester screening test and had no complaints were evaluated. Multiple pregnancies, pregnancies with fetal anomalies, pregnancies using hormone replacement therapy or assisted reproductive techniques were excluded from the study. Ethical documents, including protocols in accordance with the Declaration of Helsinki, were approved by the Istanbul Prof. Dr. Cemil Taşcıoğlu City Hospital Clinical Research Ethics Committee (Approval number: 2021/386) and written informed consent was obtained from all patients. In the hospital laboratory progesterone levels individual blood samples are collected into plain tubes and centrifuged for 10 minutes at 3000 g within 2 hours of collection. Samples are stored at –4 °C before testing. The serum progesterone level is then measured in the laboratory on the Cobas® 8000 Modular Analyzer (https://diagnostics.roche.com/tr/tr/products/systems/cobas-8000-analyzer-series-sys-128.html) according to the manufacturer’s instructions. The analytical (linear) detection range was 0.05–60 ng/mL for progesterone. The minimum detection limit was 0.05 ng/mL. The reported intra-assay and inter-assay coefficients of viability (CVs) were 2.3% and 3.2% respectively. All spiral artery Doppler examinations were performed with a 1.2–6 MHz convex abdominal probe with Mindray Resona 7 device (https://www.mindray.com/na/products/ultrasound/womens-health/resona-7/) and by Perinatologist Hicran Acar Sirinoglu. The measurements were made similarly in each case, between 11–14 weeks, in a quiet room, with the patient in the supine position and breathing levels were kept steady to prevent noise and artifacts. The methods described by Stevenson et al. [20] were used for spiral artery detection and measurements.

This study included 113 pregnancies, of which 13 patients were lost to follow-up. It was observed that 27 out of 113 pregnant women experienced problems in the perinatal and prenatal periods. 2 pregnancies were resulted in miscarriage. In addition, intrauterine growth retardation was found in 10 pregnant women, stillbirth in 1 pregnant, and preeclampsia in 5 pregnant women during pregnancy follow-up and perinatal period. Respiratory problems developed in 6 infants and jaundice in 3 infants. As a result of these, 17 of the newborns were hospitalized in the intensive care unit (1–60 days). Miscarriage is defined as pregnancy losses within 20 weeks of gestation. Intrauterine growth retardation was diagnosed in cases with a fetal weight that is below the 10th percentile for gestational age as determined through an ultrasound [21]. Pregnancies with high blood pressure (140/90 mmHg) that develops during pregnancy or during the postpartum period that is associated with overt protein in the urine or the new development of decreased blood platelets [22]. No problem was encountered in 86 of 113 pregnant women and they were recorded as non-complicated pregnancies with normal results. The study was conducted in accordance with the Principles of the Declaration of Helsinki.

The characteristics of the patients, including pregnancy outcomes and the results of serum progesterone levels, spiral artery Doppler finding are shown in Table 1. Serum progesterone and spiral artery Doppler (resistive index (RI) spiral artery, pulsatility index (PI) spiral artery and systolic/diastolic ratio (S/D)) findings of 27 complicated and 86 non-complicated pregnancies were compared. There is no statistically significant difference between serum progesterone levels and spiral artery Doppler measurement values and perinatal outcomes (Table 2).

Our study was aimed to investigate the relationship between the first trimester (11–14 weeks) serum progesterone levels, spiral artery Doppler values and pregnancy outcomes. There was no relationship detected between first trimester progesterone levels, spiral artery Doppler values and pregnancy outcomes.

Several studies have revealed that the decrease in progesterone levels in the first trimester poses a risk for a healthy pregnancy. There is currently no definitive value for diagnosing progesterone deficiency and there is no consensus on treatment in pregnant women with progesterone deficiency [23]. Some studies indicate that low serum progesterone was associated with miscarriage [24, 25, 26, 27, 28]. For this reason, exogenous progesterone supplements are often used to treat the threat of abortion [25]. However, the benefit of progesterone therapy is currently very controversial [26, 27, 28].

In the studies conducted by Özkan et al. [11] and Özkaya et al. [29] comparing spiral artery Doppler findings with fetal perinatal outcomes, no significant correlation was found between spiral artery PI and RI values and fetal perinatal outcomes. In a study by Makikallio et al. [30] they compared the results of 10 women with preeclampsia and preterm labor and 31 control group pregnant women (6th, 8th, 9th and 11th week values), and they could not find a significant difference between the spiral artery RI values between the two groups [30]. In another study first-trimester uterine artery pulsatility index (UtA-PI) and uterine artery resistive index (UtA-RI) trajectories were not significantly different between pregnancies after frozen-thawed embryo transfer (ET), fresh ET and after natural conception in both models [31]. A study by Pillai et al. [32] also suggests that the most commonly used biomarkers of serum human chorionic gonadotropin (HCG) and progesterone are not useful in predicting the outcome of a pregnancy with a live fetus. The fact that normal first trimester progesterone levels do not predict negative pregnancy outcomes may be related to the higher blood progesterone level of the therapeutic aspect of progesterone. But it does not explain the different results in patients who received progesterone therapy. A study is related to the fact that normal first trimester blood progesterone levels do not differ between positive and negative pregnancy outcomes. Low serum levels such as dual of estradiol and progesterone or estradiol alone at 7–9 weeks, $\beta$-HCG or progesterone combing estradiol at 5–6 weeks of gestation can be used better to predict miscarriage in first trimester [33]. Therefore, other factors should not be forgotten in predicting negative pregnancy outcomes.

Theoretically, first trimester serum progesterone levels and spiral artery Doppler findings were critical for the course of pregnancy, but they could not predict risks in terms of miscarriage, preeclampsia, intrauterine growth retardation, jaundice and respiratory problems. Complicated pregnancies requiring intensive care were unpredictable. Remodeling in the spiral artery takes place between 12–13 weeks and our pregnant women were evaluated between 11–14 weeks. Studies performed after an estimated 14 weeks after the completion of remodeling may be more useful in terms of evaluating spiral arteries.

This study has many strengths. All measurements were performed by one perinatology specialist. Contrary to other studies, the progesterone levels of pregnant women between 11–14 weeks who applied for the first trimester screening test were measured and their effects on pregnancy outcomes were examined. Our study had limitation that the number of patients is small.

Predicting the adverse perinatal outcomes and the risks that may be encountered during pregnancy will be beneficial both in terms of preventive medicine and in terms of preventing complications with early treatment. However, no method currently provides definitive results in predicting poor perinatal outcomes. In our study, serum progesterone levels and spiral artery Doppler findings which were measured between 11–14 weeks did not provide statistical benefit in terms of early detection of poor perinatal outcomes. We think that using more specific markers and longer follow-ups would be beneficial in this regard.

It is known that there is a strong relationship between the first trimester serum levels of progesterone, which is an important hormone for the healthy continuation of pregnancy, and the risk of miscarriage. The quantity and quality of placental and fetal circulation is determined by the changes that occur in the spiral arteries with pregnancy. Although a close relationship is known between both variables and pregnancy outcomes, no correlation could be established between them in terms of early detection of poor perinatal outcomes.

All data points generated or analyzed during this study are included in this article and there are no further underlying data necessary to reproduce the results.

RA designed the research study. FŞ, HAŞ, SÖ performed the research and analyzed the data. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript.

The retrospective study received ethical approval from the Istanbul Prof. Dr. Cemil Taşcıoğlu City Hospital Clinical Research Ethics Committee, with approval number: 2021/386. Written permission has also been obtained from the institutions where the research was conducted, and informed consent has been obtained from the patients. The study was conducted in accordance with the Principles of the Declaration of Helsinki.

We would like to express our gratitude to all those who helped us during the writing of this manuscript. Thanks to all the peer reviewers for their opinions and suggestions.

This research received no external funding.

The authors declare no conflict of interest.