Reference interval for 24-hour urinary protein excretion in uncomplicated women with twin pregnancies

The study of 24-hour urinary total protein (24-h UTP) excretion is an essential parameter for the assessment of renal function and early detection of pregnancy complications. However, data on reference interval for 24-h UTP in women with twin pregnancies are scarce.

We aimed to establish the reference interval for 24-h UTP excretion in women with twin pregnancies and compare it with that in women with singletons.

A total of 12,633 participants with singletons or twin pregnancies were recruited. They were instructed to collect standard 24-hour urinary samples. Information was extracted from the electronic medical record. The nonparametric percentile method was used to determine reference interval for 24-h UTP excretion during different trimesters in women (excluding, among others, those with preexisting renal disease, gestational or chronic hypertension, pre-eclampsia, and pregestational diabetes mellitus).

In twin pregnancies, the 24-h UTP levels expressed as medians and percentiles (5th, 95th) for each trimester were as follows: 74.2 (32.7, 195.5) mg, 104.5 (41.9, 213.1) mg, and 152.6 (46.1, 415.7) mg in the first, second, and third trimesters, respectively. A significant increase in 24-h UTP excretion was observed throughout pregnancy (all P<.001). Furthermore, the median (25th, 75th) of 24-h UTP values in the singleton group and twin groups were 70.2 (48.5, 98.3) vs 74.2 (47.3, 108.2) mg, 84.8 (58.5, 118.2) vs 104.5 (78.4, 135.6) mg, and 108.0 (73.4, 149.3) vs 152.6 (90.8, 192.8) mg in the first, second, and third trimesters, respectively. (Part of the data on singleton pregnancy has previously been published). The 24-h UTP excretion in the twin group was significantly higher than those in the singleton group in the second and third trimesters (both P<.001).

Physiological proteinuria levels were approximately doubled in late pregnancy in women with twin pregnancies compared to early pregnancy, and the upper limit of normal 24-h UTP excretion is 415.7 mg in late pregnancy. Moreover, the threshold for elevated proteinuria in twin pregnancies could be higher than in singletons from the second trimester onwards. Understanding these changes is essential for precise twin management and patient counseling.