Progesterone vs Relaxin vs Ultrasound for Breeding
· By Dr. Tang
TL;DR: Progesterone, relaxin and ultrasound aren’t competitors — they answer 3 different questions. Progesterone is the only 1 that predicts ovulation (the forward-looking “when”). Relaxin is the earliest blood “yes” for pregnancy, detectable from 21–28 days and 95–100% reliable by day 30. Ultrasound adds viability and count from about 21–25 days. Use them in sequence: time, confirm, visualise.
Breeders and clinicians sometimes treat progesterone, relaxin and ultrasound as interchangeable options. They’re not. Each answers a distinct question at a distinct point in the cycle, and using the wrong tool at the wrong time is how cycles get missed and pregnancies get mismanaged.
Three Questions, Three Tools
| Tool | Question | Timing | Nature of answer |
|---|---|---|---|
| Progesterone (serial) | When to breed? | Day 5 of heat onward | Forward-looking, hormonal |
| Relaxin (blood) | Is she pregnant? | ~21–28 days post-ovulation | Backward-looking, hormonal |
| Ultrasound | Is she pregnant / viable? | ~21–25 days onward | Backward-looking, visual |
The cleanest way to understand the three is by the direction each one looks. Progesterone looks forward — it predicts when ovulation will happen. Relaxin and ultrasound look backward — they confirm what has already happened.
Progesterone: The Only Forward-Looking Tool
This is the single most important distinction, and it gets glossed over constantly.
Progesterone is the only one of the three that predicts — it tells you ovulation is approaching before it happens. That’s what makes it irreplaceable. You can’t time a breeding with ultrasound, because ultrasound only sees structures that are already there. You can’t time it with relaxin, because relaxin only rises after pregnancy is established.
Serial progesterone — every 1–3 days from day 5 of heat — maps the rise, pinpoints the LH surge (~2–3 ng/mL) and ovulation (~5 ng/mL), and tells you when to breed (8–15 ng/mL). No other tool does this job.
Relaxin: The Earliest Blood “Yes”
Relaxin is the earliest blood-based confirmation of pregnancy, detectable from ~21–28 days after ovulation — and 95–100% reliable by day 30.
Its two defining advantages:
- It’s early — weeks before physical signs of pregnancy are obvious.
- It’s specific — placenta-derived, so it distinguishes a real pregnancy from a false pregnancy, which clinical signs alone cannot do.
Its limitation is the flip side of its specificity: relaxin tells you that she’s pregnant, but not how many or how it’s going.
Ultrasound: The Visual Layer
Ultrasound, from ~21–25 days onward, adds what neither hormone test can:
- Viability — is the pregnancy actually progressing?
- Foetal count — how many puppies to expect.
- Progression — is everything developing normally?
Ultrasound is also the tool for later-stage monitoring, when a positive relaxin has already confirmed pregnancy but you need to see what’s happening inside.
How They Fit Together
The tools aren’t a menu — they’re a sequence:
- Progesterone times the breeding. This is step one, because if the timing is wrong, nothing else matters.
- Relaxin or ultrasound confirms pregnancy ~3–4 weeks later.
- Ultrasound assesses viability and litter size as the pregnancy progresses.
A common beginner’s mistake is to skip step one — to breed “when she’ll stand” or “when the cytology looks right” and then use ultrasound to see if it worked. That approach works sometimes, and fails unpredictably, because it treats timing as a guess. The forward-looking tool exists precisely to remove that guess.
Choosing Based on Your Situation
- Frozen-semen program → quantitative progesterone is non-negotiable (12-hour sperm survival demands near-exact timing).
- Natural breeding, fresh semen → progesterone still improves consistency, but the timing tolerance is wider.
- Early confirmation → relaxin, from ~day 28.
- Viability and count → ultrasound, from ~day 25.
- Suspected false pregnancy → relaxin (pregnancy-specific) plus ultrasound for visual confirmation.
Application & Commercial Angle
Who should care: breeders and clinics building a complete reproductive workflow. Each tool answers a different question — timing, confirmation or visual assessment — so the commercial case is the combined protocol.
In practice, clinics that offer progesterone plus relaxin (and ultrasound where indicated) become the breeder’s cycle-long partner rather than a one-off vendor.
FAQ
What is the difference between progesterone, relaxin and ultrasound?
3 tools, 3 questions: progesterone answers ‘when to breed’ by tracking ovulation timing; relaxin answers ‘is she pregnant’ as the earliest blood-based confirmation; ultrasound answers ‘is it viable’ with visual evidence.
Can I replace ultrasound with relaxin testing?
For 1 question — simple yes/no pregnancy confirmation — relaxin can substitute for early ultrasound. But ultrasound adds viability, foetal count and progression that a blood test can’t show.
Can I replace progesterone with ultrasound for timing?
No — 2 timing tools differ: ultrasound visualises structures already present, but only serial progesterone can predict ovulation in advance. Progesterone is the only 1 of the 3 that answers the forward-looking ‘when’ question.
Which tool should a new breeder start with?
Start with 1 tool: progesterone, because timing is the highest-stakes step and determines whether a cycle succeeds at all. Once breeding is done, add relaxin or ultrasound for confirmation.
How do the three tools combine into a full protocol?
Use 3 tools in sequence: serial progesterone times the breeding; relaxin or ultrasound confirms pregnancy a few weeks later; ultrasound assesses viability and litter size as the pregnancy progresses.
Which is the earliest pregnancy test?
Relaxin is the earliest blood-based confirmation, detectable from ~21–28 days post-ovulation and 95–100% reliable by day 30. Ultrasound visualises structures from ~21–25 days, at a similar time point but with visual rather than hormonal evidence.
Key Takeaways
- Progesterone is the only 1 forward-looking tool — it predicts ovulation, which is why it’s the foundation.
- Relaxin is the earliest blood “yes” for pregnancy — 21–28 days, 95–100% by day 30, and pregnancy-specific.
- Ultrasound adds 3 things hormones can’t show: viability, count, progression.
- The tools are a 3-step sequence, not a menu: time with progesterone, confirm with relaxin, visualise with ultrasound.
- Skip 1 forward-looking step and everything downstream becomes a guess.
References
- dvm360. Breeding Management 102 — It’s All in the Timing (Proceedings). https://www.dvm360.com/view/breeding-management-102-its-all-timing-proceedings
- Buff S, et al. Detection of canine relaxin in maternal peripheral blood. 2001. (cited in PMC7126177)
- Kunanusont N, Punyadarsaniya D, Ruenphet S. Accuracy and precision guidelines for optimal breeding time in bitches. Vet World. 2021;14(3):585–588. doi:10.14202/vetworld.2021.585-588
This content is for educational and product-selection purposes only. It is not a substitute for veterinary diagnosis — any animal with suspected disease should be evaluated by a veterinarian. Reference ranges are assay-dependent; always use your analyzer’s validated intervals. Product specifications are as published by Migibio (Guangzhou Magic Biotech Co., Ltd.) and may change.