# Why the same disagreement stops mattering later

Six days apart again — but at twenty weeks the threshold is ten days, so the period date stands. Ultrasound dating loses precision as pregnancy advances.

**Estimated due date 2025-10-08 — currently 20w0d, with 140 days to go.**

> This is arithmetic on the dates you supplied, not medical advice. It cannot see an ultrasound, a chart, or a person. Clinical dating is done by a qualified clinician, and an estimated due date is an estimate — most births do not happen on it. Nothing here should change what you do without talking to your own clinician.

## Estimate

| | |
| --- | --- |
| Estimated due date | **2025-10-08** |
| Method | Ultrasound measurement |
| Gestational age on 2025-05-21 | 20w0d |
| Days remaining | 140 |
| Trimester | 2 |
| Estimated conception | 2025-01-15 |

## Period date vs ultrasound

| | |
| --- | --- |
| From the last period | 2025-10-08 |
| From the ultrasound | 2025-10-02 |
| Difference | 6 days |
| ACOG threshold at 16w0d to 21w6d | more than 10 days |
| Redated | no |

The two estimates differ by 6 days, which is within the 10-day threshold ACOG applies at 16w0d to 21w6d. The period-based date is retained; the ultrasound is consistent with it rather than replacing it.

## Milestones

| Date | Gestational age | Milestone | Note |
| --- | --- | --- | --- |
| 2025-04-08 | 13w6d | End of first trimester | Also the last point at which ultrasound dating is at its most accurate. |
| 2025-05-07 | 18w0d | Anatomy scan window opens | The detailed anatomy scan is usually done between 18 and 22 weeks. |
| 2025-06-04 | 22w0d | Anatomy scan window closes | After this the standard views become harder to obtain. |
| 2025-07-15 | 27w6d | End of second trimester | The third trimester begins the following day. |
| 2025-09-17 | 37w0d | Early term begins | ACOG classifies 37w0d to 38w6d as early term, not full term. |
| 2025-10-01 | 39w0d | Full term begins | Full term runs 39w0d to 40w6d. |
| 2025-10-08 | 40w0d | Estimated due date | A reference point, not a prediction. Most births are not on this day. |
| 2025-10-22 | 42w0d | Post-term begins | From 42w0d a pregnancy is classified post-term. |

## Read before relaying this

### Gestational age counts from the last period, not from conception (caution)

This is the most misunderstood fact in the subject. The 40-week clock starts at the first day of the last menstrual period, roughly two weeks BEFORE conception. So at 20w0d the pregnancy is about 18w0d since conception. Weeks 1 and 2 of a pregnancy are, by this convention, before there is a pregnancy.

**What to do:** When comparing against a known conception or transfer date, use that method instead — it does not go through the two-week offset.

### The due date is a reference point, not a prediction (caution)

Only a small minority of births happen on the estimated date. The useful output is the term windows — early term from 37w0d, full term from 39w0d — not the single day.

### The two estimates agree within 10 days (info)

The two estimates differ by 6 days, which is within the 10-day threshold ACOG applies at 16w0d to 21w6d. The period-based date is retained; the ultrasound is consistent with it rather than replacing it.

### Not medical advice (info)

This is arithmetic on the dates you supplied, not medical advice. It cannot see an ultrasound, a chart, or a person. Clinical dating is done by a qualified clinician, and an estimated due date is an estimate — most births do not happen on it. Nothing here should change what you do without talking to your own clinician.

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Canonical URL: https://due-date.gumballtools.com/dating/redating-later-in-pregnancy
JSON API: `GET https://due-date.gumballtools.com/api/v1/estimate?method=lmp&date=YYYY-MM-DD`
MCP endpoint: `https://due-date.gumballtools.com/api/mcp`