Dog and Cat Breeding Management: Progesterone to Ultrasound
· By Dr. Tang
TL;DR: Breeding management runs on two questions — “when to breed?” and “is she pregnant?” — and three tools answer them: progesterone for timing, relaxin for early pregnancy confirmation, and ultrasound for visualisation. The timing decision is the highest-stakes step, and it demands a quantitative number, because the difference between 2 ng/mL and 8 ng/mL is the difference between a missed window and a full litter.
Successful canine and feline breeding depends on getting the timing right and confirming the result early. Three tools carry most of the workload — progesterone for timing, relaxin for pregnancy confirmation, and ultrasound for visualisation — and they answer different questions at different points in the cycle.
The Two Questions Breeding Diagnostics Answer
Every breeding program, whether a single litter or a large kennel, runs on two questions:
- When is the optimal time to breed? — answered by tracking progesterone, which pinpoints ovulation and the fertile window.
- Is she pregnant? — answered by relaxin testing or ultrasound, which confirm pregnancy weeks before it is physically obvious.
No single test answers both. A well-run program uses them in sequence.
Progesterone: Timing Ovulation
Progesterone is the foundation of breeding timing in the bitch. It rises in a predictable, gradual pattern through the cycle — but the details of that pattern carry more information than most people use.
The classic reference points, drawn from large-scale clinical studies:
| Progesterone | Event |
|---|---|
| <1 ng/mL | Baseline (anoestrus/early proestrus) |
| 2–3 ng/mL | LH surge approaching (a large-scale study found a mean of 2.7 ng/mL at surge) |
| ~5 ng/mL | Ovulation |
| 8–15 ng/mL | Optimal breeding window |
| >20 ng/mL | Late fertile period |
Two nuances matter in practice. First, ovulation occurs roughly 48–72 hours after the LH surge, when progesterone reaches about 4–10 ng/mL. Second, the bitch ovulates immature oocytes that need another 48–72 hours to mature in the oviduct — which is why the true fertile window lands four to six days after the LH surge, not at ovulation itself.
The Rate of Rise Matters More Than You’d Think
Here’s a finding most breeding guides skip, and it’s one of the most actionable. Research on over 1,400 breeding cycles found that the rate of progesterone rise, not just the absolute value, predicted litter size: a slow rise paired with frozen semen produced an average of 3.9 puppies, versus 5.6 with a fast rise.
The practical takeaway: when you’re tracking a bitch’s progesterone serially, don’t just note “it’s rising” — note how fast. A slow climber may need a different insemination strategy (or a shift in semen type) than a fast one.
Semen Type Changes the Timing Precision You Need
The type of semen dictates how precisely you must time insemination, because sperm survival varies dramatically:
| Semen type | Survival in the bitch |
|---|---|
| Fresh | 4–6 days |
| Chilled | ~3 days |
| Frozen | ~12 hours |
Fresh semen is forgiving — a breeding a day off either way still lands. Frozen semen is unforgiving — a 12-hour survival window means the insemination must hit the fertile window almost exactly. This is precisely why frozen-semen programs demand a quantitative progesterone number, not a “she’s in heat” guess.
Relaxin: Confirming Pregnancy
Relaxin is a hormone produced by the placenta in pregnant bitches. Its value is that it is pregnancy-specific: it’s essentially absent in non-pregnant dogs and becomes detectable in blood around 21–28 days after ovulation.
The clinical literature dates relaxin detection to as early as 25 days after ovulation (Buff et al., 2001), with detection more reliable after day 28 — and by day 30 post-ovulation, a validated test detects relaxin in 95–100% of pregnant dogs.
This makes relaxin the earliest blood-based confirmation of pregnancy — earlier and more definitive than physical signs, and complementary to ultrasound. A relaxin test answers “is she actually pregnant?” weeks before whelping, which matters for:
- Planning — nutrition, monitoring, and whelping preparation.
- Managing expectations — confirming pregnancy (or its absence) early.
- Detecting problems — distinguishing true pregnancy from false pregnancy.
Ultrasound: Visualising the Pregnancy
Ultrasound is the imaging complement. From around day 21–25 after ovulation, gestational structures become visible, and later scans can confirm foetal viability and estimate litter size.
Ultrasound answers the “is she pregnant and how is it going?” question with visual evidence — but it does not give the hormonal timing information that progesterone provides, nor is it as early as relaxin for a simple yes/no.
How the Three Tools Fit Together
| Tool | Question answered | Timing | Best use |
|---|---|---|---|
| Progesterone (serial) | When to breed | From ~day 5 of heat, every 1–3 days | Ovulation timing, AI scheduling |
| Relaxin | Is she pregnant? | From ~day 21–28 post-ovulation | Early pregnancy confirmation |
| Ultrasound | Is she pregnant / viable? | From ~day 21–25 onward | Visual confirmation, foetal assessment |
A typical well-run cycle uses progesterone to time the breeding, then relaxin or ultrasound a few weeks later to confirm the result.
Why Quantitative Progesterone Matters
Progesterone timing is the clearest possible case for quantitative over qualitative testing. A colour-change “progesterone present” strip cannot tell you whether you’re at 2 ng/mL or 8 ng/mL — and that distinction is precisely what determines breeding success.
The conversion to remember: ng/mL × 3.18 = nmol/L, and vice versa. Different analyzers and reference labs report in different units, and a misread unit is a misread cycle.
An in-house quantitative immunoassay analyzer returns the actual number in minutes, letting you time insemination the same day rather than waiting for a reference lab — which matters most when the cycle is moving fast and the window is measured in hours.
Application & Commercial Angle
Who should care: breeders timing inseminations and clinics offering reproductive services. The breeder gets fewer wasted cycles and better timing decisions; the clinic gets a repeatable, billable reproductive protocol.
In practice, the value is precision and continuity: progesterone numbers set the breeding window, and relaxin or ultrasound confirms the outcome later. That makes the clinic the breeder’s monitoring partner across the whole cycle rather than a one-off ultrasound stop.
FAQ
What tests are used to manage canine breeding?
Three tools cover the whole breeding cycle: serial progesterone measurement to time ovulation (breed at 8–15 ng/mL), relaxin testing to confirm pregnancy (reliable from day 28–30), and ultrasound to visualise it. Progesterone answers ‘when to breed’; relaxin and ultrasound answer ‘is she pregnant’.
When should I start progesterone testing?
Progesterone monitoring typically begins around day 5 of the heat cycle, or when vaginal cytology shows the onset of oestrus. Testing is then repeated every 1–3 days, with frequency increasing as the progesterone rise approaches.
How early can pregnancy be confirmed in a dog?
Relaxin becomes detectable in blood around 21–28 days after ovulation, making it the earliest blood-based confirmation of pregnancy. Ultrasound can visualise gestational structures from around day 21–25, giving visual confirmation at a similar time point.
Why does ovulation timing require quantitative progesterone?
Because the decision lives in 3 numbers: progesterone around 2–3 ng/mL signals the LH surge, ~5 ng/mL signals ovulation, and 8–15 ng/mL marks the optimal breeding window. A qualitative strip can’t distinguish these — only a quantitative measurement can.
Does the rate of progesterone rise affect litter size?
Research on over 1,400 breeding cycles found that a slow progesterone rise with frozen semen produced an average of 3.9 puppies, versus 5.6 with a fast rise. The rate of rise, not just the absolute value, carries clinical information.
How long do sperm survive in the bitch?
Fresh semen survives roughly 4–6 days, chilled semen about 3 days, and frozen semen only about 12 hours. This is why frozen-semen insemination demands far more precise ovulation timing than fresh-semen natural breeding.
Key Takeaways
- Breeding runs on two questions — timing (progesterone, breed at 8–15 ng/mL) and pregnancy (relaxin from day 21–28, or ultrasound) — answered by three tools working together.
- Progesterone reference points: LH surge ~2–3 ng/mL, ovulation ~5 ng/mL, optimal window 8–15 ng/mL.
- The rate of progesterone rise predicts litter size — a slow rise with frozen semen averaged 3.9 puppies vs 5.6 for a fast rise.
- Semen type changes the timing precision you need: frozen semen survives only ~12 hours.
- Relaxin confirms pregnancy from ~21–28 days, with 95–100% detection by day 30.
References
- Buff S, et al. Detection of canine relaxin in maternal peripheral blood (ReproCHEK ELISA). 2001. (cited in J Zoo Wildl Med, PMC7126177)
- dvm360. Breeding Management 102 — It’s All in the Timing (Proceedings). https://www.dvm360.com/view/breeding-management-102-its-all-timing-proceedings
- 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
- Comparison of three progesterone quantification methods using blood samples drawn from bitches during the periovulatory phase. PMC8924395.
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.