What is relaxin?

Relaxin is a protein hormone that is released from both the placenta and in many species, also from the ovaries. It is primarily secreted from the placenta in cats and dogs, making it a useful test in pregnancy diagnosis.

What does relaxin do?

As it's name might suggest, it has a role in softening of the cervix around the time of parturition relaxing this otherwise firm structure to permit the delivery of babies. In some species, like the guinea pig, it also relaxes the ligaments to the pelvis, allowing the bones to separate temporarily to permit delivery of babies. Around the times or parturition, there may be as much as 1-2 cm of space between the pubic bones in the guinea pig sow!!!

How is relaxin measured?

There are laboratories that can measure serum levels. More commonly, there is an enzyme-linked absorbent assay (ELISA) kit that can be used in clinic to measure a critical range of relaxin.

What is the use of relaxin?

Relaxin tests can be used after 25 days of gestation to determine if pregnancy exists. Non-pregnant dogs and cats should not have placentae to produce this hormone, so when used correctly it can be a very specific test. This test can be run in a matter of 30 minutes off of a single blood sample, drawn by our experienced and caring technicians.

What are the limitations of this test?

Well, first it really can't be used before 25 days gestation in the dog (20-29 days in the queen), so this means either ovulation timing (see what is progesterone) was used to determine the date of gestation, or we need to underestimate the length of gestation from the last possible date of breeding. Testing too early can produce a false negative result and cells from the placenta have not secreted enough relaxin yet. False positive tests can also occur. These may happen as a result of lab error or if a bitch/queen was pregnant but may have lost the pregnancy within the last week. Relaxin levels may remain elevated for days after pregnancy loss has occurred. It is generally recommended when using this as a test of pregnancy that 2 tests are run at least 7-10 days apart. Or that an ultrasound follows a positive relaxin test.

How does this test compare to ultrasound for pregnancy diagnosis?

Note that since pregnancy is most commonly diagnosed at 20-24 days (17-20 days in cats) by ultrasound, the relaxin test is neither more sensitive nor specific (no more accurate and may be less) than an abdominal ultrasound. Ultrasound may also give the veterinarian information about the health and development of the babies as well as be used to estimate their fetal age to help predict a whelping/queening date. Ultrasound is the recommended test for bitches/queens that have a history of pregnancy loss in previous cycles.

How does this test compare to abdominal radiographs for pregnancy diagnosis?

Since fetal bones are not normally radiopaque before 40-45 days of gestation (more like 50 days in the queen), are there are some concerns for x-ray radiation on developing fetuses (and their gonads) before 40 days gestation, x-rays are not normally taken of the pregnant bitch before 50 days gestation. As pregnancy is typically 58-72 days, this can mean waiting until the last 1-2 weeks before a diagnosis is made. This may not yield any information on pregnancy loss, especially before 30 days of gestation.

My dog may be pregnant. She was bred about 3 weeks ago. What is the best test to determine if she is pregnant?

If the bitch was bred off of a single breeding on a timed breeding (by surgical or TCI breeding) with frozen semen. We could see her today for an ultrasound to determine if the breeding was successful. We could also wait another 5-7 days to run a relaxin test or plan to take abdominal radiographs in 4 weeks (depending on the history of the bitch, and the owner's plans and desires as far as managing this pregnancy). With other methods of breeding- using fresh or chilled semen, there may be a factor of error of several days, so I would choose the date of pregnancy diagnosis based off of the last date of breeding. For example, a bitch that stood to be bred naturally 3 weeks ago, may have had semen in her reproductive tract that could remain viable for up to 1 week (so really only about 2 weeks from the point of conception if eggs were viable at the end of that week). I would predict that ultrasound could be used in another 7-10 days to diagnose pregnancy. Relaxin could be used in another 2 weeks and radiographs could be taken in another 5 weeks.

It's probably best to check with NHAH to determine when the best time to evaluate for pregnancy diagnosis, and what method offers the information the owner needs.

My cat may be pregnant. She was in heat a week ago and with an intact tom cat. What is the best way to determine is she is pregnant?

Queens are considerably different from the bitch (as mentioned previously). Queens are induced ovulators (only infrequently spontaneous ovulators), and so mating is what normally drives the hormones to induce release of fertilizable eggs. If the mating occurred too early in the queen's estrual period, not enough matings occurred during her estrus or the tom was sterile, the queen will ovulate but fail to become pregnant. After 5 days, her progesterone levels will rise whether pregnant or not, but the non-pregnant queen will have an early fall in progesterone levels usually by days 40-45. The pregnant queen would maintain progesterone to the end of pregnancy.

Progesterone is not an ideal test for pregnancy, but if checked twice during the queen's diestrus (once after 5 days to confirm an elevation, and then again after 40 days to confirm a drop to baseline), then it can signal a non-fertile mating. Typically an interestrual period of 40-50 days (queen not in heat during breeding season for 40 -50 days) is about as suggestive.

Queens may be significantly easier to palpate the pregnant uterus per abdomen than certain dogs (think small queen's abdomen as compared to expansive English Mastiff abdomen). This is normally done at 21-25 days post-mating and can still be fraught with false positive findings- feces in the colon can be misleading and sometimes feel like fetuses in the uterus.

Ultrasound after 17-20 days post-mating can be used to confirm pregnancy, estimate litter size and give some indication of fetal viability.

Radiographs are usually taken after 38-40 days post-mating to identify calcification of the fetal skeletons.

Relaxin can be used after 20-29 days gestation in the queen. The canine relaxin assay has been shown to be accurate in this species.

Depending on the owner's interests in this litter, I would proceed with a test as per the amount of information needed in a time frame that is suitable.

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