The main characteristics of each registry are summarized below and listed in Table 1.

PARTOUT was a cohort study established in 2017 involving all eight kidney transplant centers in the Netherlands. It included all pregnant KTRs in the country from 1971 until 2017, when the data collection was completed. Participants were identified via the National Organ Transplant Registry, where all Dutch transplant patients are registered. Further consultation with obstetricians and transplant nephrologists in the participating centers contributed to finalizing participant inclusion. Data were collected from medical charts at the transplant centers by dedicated members of the research team. The highest number of pregnancies reported in a year was 16 in 2015, while the lowest was 8 in 2017 (Fig. 1). In 2024, the PARTOUT network evolved into the Pregnancy, Genetics & Nephrology (PREGeNEPH) registry that actively collects data prospectively on pregnancy in KTRs and chronic kidney disease patients.

Number of pregnancies in KTRs reported annually by each registry

PARTOUT included all pregnancies post-KTx in the country, thus providing comprehensive national coverage. Moreover, the data were collected from medical charts, hence avoiding self-reported data and lowering selection bias. PARTOUT also conducted the largest study worldwide on how pregnancy post-KTx affects long-term estimated glomerular filtration rate (eGFR) [9].

Limitations were that data collection came to an end on December 31, 2017 and only retrospective data were collected. The data were gathered from a long time interval and missing data could not be retrieved. Relying on input from healthcare professionals to determine patient inclusion and excluding pregnancies from the analysis because of missing data could have introduced bias.

ANZDATA is a bi-national, ongoing, prospective registry that annually gathers clinical data from dialysis and transplant units in Australia and New Zealand [6, 7]. ANZDATA captures maternal and paternal events reported while receiving kidney replacement therapy ([KRT]; chronic dialysis or kidney transplantation). ANZDATA has 100% capture of KRT recipients. Parenthood events, reported via the treating transplant unit, have been captured since 1968, but a parenthood form was introduced in 2001 to formalize this data collection process. Paper surveys and online forms with obligatory fields have been utilized for data collection. During the last ten years the highest number of annually reported pregnancies post-KTx was 30 pregnancies in 2014, while the lowest was 15 in both 2016 and 2018 (Fig. 1).

ANZDATA is the oldest among the three registries and started collecting data prospectively. Additionally, registry staff monitor data completeness across various variables, further adjusting the data collection via liaison with treating units.

Regarding the limitations of the registry, surveys are completed on paper or online, though not all items are mandatory to enter, and fields may be left blank. The data collectors are renal unit staff who may not have the entire pregnancy record available if obstetric care occurred at a different site, resulting in missing data [7]. Case capture has been previously validated against birth registries and is higher than 75% [7].

TPRI is a voluntary pregnancy registry that has been actively collecting data since 1991 and includes pregnancies from 1968 onward. In 2010, TPRI began collecting data prospectively in addition to retrospective data collection. Solid organ transplant recipients who have had or fathered a pregnancy are included. The highest number of pregnancies post-KTx included per year was 150 pregnancies in 2020 (Fig. 1), when the COVID-19 pandemic began. Registration takes place via a health provider or self-enrollment, while interviews take place at enrollment, post-delivery, and then every one to two years as long as recipients can be reached. In 2016, TPRI expanded to include participants from all countries [5]. Participants are excluded if they cannot communicate in English or provide a translator. There are 14 Australian KTRs with 24 pregnancies and 1 Dutch KTR with 4 pregnancies included in TPRI.

Strengths of TPRI include long-term follow-up and global inclusion. Data collection on non-KTR pregnancies after solid organ transplantation leads to inclusion of different end-stage organ failure groups that could be at higher risk of adverse pregnancy or graft outcomes than KTRs.

However, limitations include its voluntary nature, which makes it susceptible to selection bias. Furthermore, the number of participants from countries outside the USA is low, incompletely filling the need for participants from different countries for more inclusive data capture. Lastly, when data are collected from participants retrospectively, recall bias can be introduced.

Pregnancy, neonatal, and graft outcomes are listed in Table S1. Regarding pregnancy outcomes, PARTOUT includes live births only past the 20th gestational week. ANZDATA and TPRI report miscarriages before the 20th gestational week. All three registries report stillbirth defined as after the 20th gestational week. Ectopic pregnancies are reported only by TPRI, and termination rates by ANZDATA and TPRI. Reported live birth rates among PARTOUT, ANZDATA, and TPRI ranged between 75–93%. Mean gestational age was approximately 35 weeks for all registries, while mean birth weight was 2383, 2360 (median), and 2551 g for PARTOUT, ANZDATA, and TPRI, respectively. Approximately one third of the participants had preeclampsia across all registries. Graft loss was reported in 23% of the PARTOUT recipients at a median time of 6.44 years post-delivery. In ANZDATA, 27% of KTRs experienced graft loss with a median follow-up of 8.08 years postpartum, while 33% of TPRI participants had graft loss at their last follow-up 14.4 years post-delivery.