Content Reviewers:Rishi Desai, MD, MPH
The fetal membranes are also called the chorioamniotic membranes, and they make up the amniotic sac.
Inside the amniotic sac, there’s amniotic fluid, so throughout pregnancy, the fetus develops in a bubble of fluid that protects it from shock and infections.
When the amniotic sac ruptures, it releases the amniotic fluid, which is why a lot of individuals describe it as “their water breaking”.
Normally, this occurs right before labor, or sometimes even during labor, and it’s associated with uterine contractions.
If the membranes rupture in the absence of uterine contractions, that has different names depending on when it happens during the pregnancy.
If they rupture after 37 weeks, that’s simply called premature rupture of membranes, or PROM for short.
So premature doesn’t have anything to do with a premature delivery, but rather with it occuring before labor, in the absence of uterine contractions - which is why, sometimes, the term prelabor rupture of membranes is also used.
If the membranes rupture before 37 weeks gestation, that’s called preterm premature rupture of membranes, or pPROM for short.
And no matter when it happens during pregnancy, if more than 18 hours pass between rupture of membranes and delivery of the fetus and placenta, that’s called prolonged rupture of membranes.
Risk factors for PROM and pPROM include PROM or pPROM in a previous pregnancy, genital or urinary tract infections, smoking, as well as polyhydramnios, which means there’s too much amniotic fluid, and abdominal trauma.
Diagnosing premature rupture of membranes requires a speculum exam and specific tests.
Bear in mind that a digital examination of the cervix should be avoided, as it can increase the risk of infection, and may precipitate labor in females with pPROM.
Ok, now the speculum exam shows fluid pooling in the posterior vaginal fornix.
The fluid may be clear, or there may be blood or meconium in it, which is actually not a cause for concern.
To confirm that the fluid is amniotic fluid, the nitrazine and the fern test are done.
With the nitrazine test, some fluid from the vagina is placed on a PH sensitive strip.
With a positive nitrazine test, the color of the strip changes to dark blue, which indicates a PH greater than 7.1, that’s consistent with amniotic fluid.
With the fern test, a bit of fluid is placed on a slide, and left for a couple of minutes to dry, and then examined under a microscope.
If a characteristic ferning pattern shows up, that’s a positive fern test.
If no fluid is seen in the posterior vaginal fornix, first the individual can be asked to cough and press on the uterine fundus, which may enhance amniotic fluid flow through the cervical opening.
If pooling is still not seen, an ultrasound is done to assess amniotic fluid volume - which should be low in the context of PROM and pPROM.
The level of amniotic fluid is expressed as the amniotic fluid index, or AFI, and oligohydramnios is when the AFI is less than or equal to 5 centimeters, which confirms premature rupture of membranes.
If AFI is low-normal, meaning 6 or 7 centimeters, a commercial test is ordered to confirm.
There are 3 options: the PAMG-1 test, which looks for trace amounts of placental alpha-microglobulin-1 protein in vaginal fluid, the IGFBP-1 test, which looks for placental protein 12 in vaginal fluid, and a combined test, that looks for both placental protein 12 as well as alpha-fetoprotein.
Now, once PROM or pPROM is confirmed, fetal status is also assessed.
The ultrasound determines fetal position and gestational age, and fetal well-being is assessed with a non-stress test, or NST.
An NST is a 20 minute recording of fetal heart rate using a cardiotocograph - also called an electronic fetal monitor.
On a normal NST, fetal heart rate varies between 110 and 160 beats per minute, and has at least 2 accelerations - defined as increases of 15 beats per minute in fetal heart rate, lasting for more than 15 seconds in pregnancies beyond 32 weeks, or increases of 10 beats per minute lasting for at least 10 seconds in pregnancies below 32 weeks.
A lower heart rate, fewer or shorter accelerations mean the NST is non-reactive, and may signify fetal distress.
And finally, maternal work-up includes screening for sexually transmitted infections - which can be both a cause or a consequence of premature rupture of membranes.