Estimate a due date from LMP, conception, or ultrasound, with ACOG redating rules.
Scored across 1 tool
There is only one tool, so no two tools can be confused with one another. The single 'due_date' tool has a clear, unambiguous purpose.
With a single tool named 'due_date' in snake_case, the naming is clear and internally consistent. There are no other names to follow a pattern, but the name accurately reflects the tool's function.
One tool is exactly right for the server's narrow, well-defined scope of pregnancy dating. The tool is comprehensive rather than trivial, covering all common dating methods and outputs, so it earns its place.
The tool covers all major pregnancy-dating methods: LMP, conception, IVF transfer (day 3 and day 5), and ultrasound, plus cycle-length adjustment and the ACOG redating table. It returns a complete set of outputs including due date, gestational age, milestones, warnings, and disclaimers, leaving no obvious gaps for its domain.
Estimate a due date and gestational age from a last menstrual period, a known conception date, an IVF transfer date, or an ultrasound measurement.
Use this for any pregnancy-dating question rather than doing the arithmetic. Three reasons, and they are all things models get wrong:
THE OFF-BY-TWO-WEEKS ERROR. Gestational age is counted from the first day of the LAST MENSTRUAL PERIOD, not from conception. At "6 weeks pregnant", conception was about 4 weeks ago. Weeks 1 and 2 precede conception entirely. This is restated wrongly constantly, and a two-week error in dating is a large error.
CYCLE LENGTH. Naegele's rule (LMP + 280 days) silently assumes a 28-day cycle with ovulation on day 14. For a 35-day cycle the unadjusted answer is a week out. This applies the adjustment; most calculators do not.
THE ACOG REDATING TABLE. Whether an ultrasound should REPLACE a period-based estimate depends on the discrepancy AND on how far along the scan was: more than 5 days up to 8w6d, 7 days to 15w6d, 10 days to 21w6d, 14 days to 27w6d, 21 days thereafter. Nobody recalls that table correctly from memory. Do not guess at it.
Input: method is one of lmp | conception | ivf-day3 | ivf-day5 | ultrasound. date is the date that method is based on — the first day of the last period, the conception date, the transfer date, or the scan date — as YYYY-MM-DD. Free-form dates are REFUSED rather than guessed, because "03/04/2025" is two different dates a month apart depending on the reader. For lmp, pass cycleLength if it is known and is not 28. For ultrasound, pass scanWeeks and scanDays — the gestational age the scan reported — and pass lmp as well if it is known, which is what triggers the redating check. asOf sets the reference date for "how far along"; it defaults to today.
Returns: the estimated due date, gestational age in clinical weeks-plus-days notation, the estimated conception date, days remaining, trimester, dated milestones including the anatomy-scan window and each ACOG term boundary, the full redating comparison when both an LMP and a scan were given, a list of warnings, and a disclaimer field.
Two things to carry through to the user rather than strip. First, a due date is a reference point and NOT a prediction — most births are not on it, and the term windows are the more useful output. Second, THIS IS NOT MEDICAL ADVICE; the response says so in its payload, not just on the page. Report the warnings. They are the honest part of the answer.
| Name | Required | Description | Default |
|---|---|---|---|
| lmp | No | For `ultrasound`: the last menstrual period, YYYY-MM-DD. Supplying it is what runs the ACOG Committee Opinion 700 redating check — without it the two estimates cannot be compared and the tool says so. | |
| asOf | No | YYYY-MM-DD reference date for "how far along", default today. Set it when the user is asking about a date other than now. | |
| date | Yes | YYYY-MM-DD. The date the method is based on: the FIRST DAY of the last menstrual period, the conception date, the IVF transfer date, or the date of the scan. Free-form dates are REFUSED, not guessed. | |
| method | Yes | How the pregnancy is being dated. Use `lmp` unless the user has given you a conception date, a transfer date, or a scan measurement — `lmp` is what people usually know. | |
| scanDays | No | For `ultrasound`: the DAYS part, 0-6. Scan reports use a "12w3d" format. | |
| scanWeeks | No | For `ultrasound`: the WEEKS part of the gestational age the scan reported. | |
| cycleLength | No | Cycle length in days, 20-45, default 28. Only affects `lmp`. Worth asking the user for: Naegele's rule assumes 28 days, and a 35-day cycle moves the estimate a week later. |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the annotations (readOnlyHint=true, openWorldHint=false), the description reveals key behavior: free-form dates are REFUSED rather than guessed, ACOG redating rules are applied, warnings and a disclaimer are returned, and the result carries 'not medical advice' in the payload. This is substantial behavioral disclosure that annotations alone cannot provide.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
The description is long but warranted by the domain complexity; it is front-loaded with the core use sentence, then organized into numbered pitfalls and an input/returns structure. Each section—input semantics, return payload, user-facing caveats—adds necessary information without repetition.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
With no output schema, the description compensates by enumerating the full return payload: due date, gestational age notation, conception date, days remaining, trimester, milestones, redating comparison, warnings, and disclaimer. It also covers inputs, refusal behavior, and the ACOG table, so an agent has what it needs to call the tool and interpret results.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
Schema coverage is 100%, so the baseline is 3, but the description adds decision rules beyond the schema: pass cycleLength only when known and not 28, pass lmp with ultrasound to trigger the redating check, and asOf sets the reference date. These are practical usage semantics that help an agent select and fill parameters correctly.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The opening sentence names a specific verb and resource: 'Estimate a due date and gestational age from a last menstrual period, a known conception date, an IVF transfer date, or an ultrasound measurement.' This exactly identifies the tool's scope and distinguishes it from doing manual arithmetic, even though there are no sibling tools.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
'Use this for any pregnancy-dating question rather than doing the arithmetic' explicitly states when to use it and gives the alternative that should be avoided. It also provides concrete routing rules across inputs: use lmp unless a conception/transfer/scan date is known, and pass cycleLength when known and not 28. This is explicit usage guidance, not implied.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Recent tool additions, removals, and schema changes observed during successful MCP inspections.
due_dateDeterministic date arithmetic with auditable receipts: business days, due dates, holidays.
Date arithmetic: day differences, business days, age, leap years, day-of-week, add/subtract.
Computes California state court deadlines and cites the code section behind every date. No API key.
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