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OG37.6 | Outlet Forceps, Vacuum and Breech Delivery Observation — SDL Guide

Learning Objectives

• Identify the clinical indications and prerequisites for outlet forceps, vacuum extraction, and vaginal breech delivery
• Describe the anatomical assessments required before assisted delivery — station, position, and engagement
• Follow the sequential steps of an outlet forceps delivery and a vacuum extraction as an informed observer
• Describe the types of breech presentation and the specific manoeuvres used in assisted vaginal breech delivery
• Interpret intraoperative findings to assess progress during instrumental delivery
• Recognise maternal and neonatal complications of assisted delivery and observe the team's immediate response
• Document an observed assisted delivery accurately in the logbook

INSTRUCTIONS

Assisted delivery — instrumental delivery with forceps or vacuum, and vaginal breech delivery — represents the most complex and time-sensitive decision-making in intrapartum obstetrics. As a final-year student you are required to observe and assist (OG37.6, SH level). This module prepares you for that experience by explaining the indications, the anatomy, the procedural steps, and the complications of all three forms of assisted delivery. Read this module before your labour-ward posting and keep it in mind every time a woman in the second stage of labour is not progressing.

References

• DC Dutta's Textbook of Obstetrics, 9th edition, Chapter 32 — Forceps Delivery; Chapter 33 — Vacuum Extraction; Chapter 38 — Breech Presentation (textbook)
• Williams Obstetrics, 26th edition, Chapter 29 — Operative Vaginal Delivery; Chapter 28 — Breech Delivery (textbook)
• RCOG Green-top Guideline No. 26 — Operative Vaginal Delivery, 2011 (updated 2023) (guideline)
• RCOG Green-top Guideline No. 20b — Management of Breech Presentation, 2017 (guideline)
• Hannah ME et al. TERM Breech Trial — Planned C-section vs planned vaginal birth for breech presentation at term. Lancet 2000 (evidence)

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Version 2.0 | NMC CBUC 2024

Why Assisted Delivery? Indications and Prerequisites

Every assisted delivery involves a risk-benefit decision in which the potential benefit of expediting delivery — rescuing a compromised fetus or relieving a mother who cannot complete the second stage — must clearly outweigh the risks of maternal and neonatal trauma from the instrument or manoeuvre chosen. This decision is not made lightly, and it is never made by a single person: it requires the senior obstetrician present to confirm that the clinical situation meets the threshold for instrumental intervention, that the anatomy is favourable, and that all technical prerequisites are in place before the first instrument is placed. As an observer, your preparation for this moment means you should be able to run through the prerequisites mentally as the registrar runs through them clinically — before the procedure begins, not during it. A student who arrives at the scrub table understanding why the decision was made to use forceps today for this patient is positioned to learn from every step that follows.

Indications for assisted vaginal delivery (forceps or vacuum):
- Fetal compromise in the second stage — CTG changes suggesting fetal distress (late decelerations, prolonged bradycardia) when the head is in a position and at a station where safe instrumental delivery is feasible
- Prolonged second stage — exceeding the defined time limits (>2 hours nullipara, >1 hour multipara without epidural; longer limits apply with epidural) without progress despite adequate uterine contractions and maternal effort
- Maternal exhaustion — the mother is unable to push effectively, especially relevant in severe cardiac or respiratory disease where prolonged Valsalva is dangerous
- Prophylactic shortening of second stage in conditions where pushing is contraindicated — severe pre-eclampsia, cardiac disease, intracranial lesions

Prerequisites for any instrumental vaginal delivery (the 'ABCDE' mnemonic used clinically):
- A — Adequate analgesia: instrumental delivery without adequate anaesthesia/analgesia is unacceptable; perineal block, pudendal block, or existing epidural
- B — Bladder empty: an indwelling catheter empties the bladder and reduces risk of bladder injury
- C — Cervix fully dilated: attempted delivery through an incompletely dilated cervix causes cervical laceration
- D — Descent and position confirmed: exact position and station must be known before instrument placement; never apply forceps to an unengaged head
- E — Equipment ready: experienced operator, neonatal resuscitation team available, CS available as immediate backup if trial of instrumental delivery fails

Classification of forceps by station:
Outlet forceps (most safe — head at perineum, visible without parting labia), Low forceps (head at or below +2 station, rotation ≤45 degrees), Mid forceps (head above +2 station — rarely used today, high risk). Nearly all instrumental deliveries in current practice are outlet or low forceps.

Provided image

Anatomy and Assessment Relevant to Assisted Delivery

Safe instrumental delivery begins with a careful vaginal examination to confirm four key findings: the exact station, the exact position, complete dilatation, and the absence of caput or moulding that might indicate CPD. An error in any of these assessments before instrument placement can result in catastrophic injury to the mother and baby. This is why the senior obstetrician or registrar personally performs the assessment — it is not delegated.

Station assessment is made by vaginal examination, identifying the ischial spines by palpation (they are bony prominences felt on each lateral pelvic wall at the mid-level of the pelvis) and estimating the distance of the leading part of the head relative to those spines in centimetres. The vertex at the level of the ischial spines = 0 station; below the spines = positive station (+1 to +5, where +5 = on the perineum); above the spines = negative station. Outlet forceps requires the head to be visible at the introitus (+5 station or nearly) or at least at +3 station with scalp visible.

Position assessment requires identifying the fetal occiput relative to the maternal pelvis quadrants. The posterior fontanelle (small, triangular, three suture lines) identifies the occiput; the anterior fontanelle (larger, diamond-shaped, four suture lines) identifies the bregma. OA (occiput anterior, 12 o'clock position) = ideal for forceps delivery. OP (occiput posterior, 6 o'clock) = the head is deflexed; forceps traction in this position must account for the need to flex the head. OT (occiput transverse) = requires rotation.

Caput succedaneum (oedematous swelling of fetal scalp soft tissue due to pressure against the cervix) and moulding (overlapping of fetal skull bones at suture lines due to compressive forces) are assessed by the degree of bony overlap and soft tissue swelling palpated during vaginal examination. Severe moulding (+++) with caput suggests significant CPD — an important contraindication to instrumental delivery even if the station appears favourable.

Pelvic adequacy is clinically assessed by noting whether the pelvis is android, gynaecoid, anthropoid, or platypelloid in shape (Caldwell-Moloy classification) and whether clinical measurements are adequate. In an obstructed labour with severe moulding and prominent ischial spines, any attempt at instrumental delivery is dangerous.

Fetal Head Station and Occiput Position on Vaginal Examination

Outlet Forceps Delivery — Step-by-Step Observation

Outlet forceps delivery with Wrigley's forceps (the standard instrument for outlet application) follows a logical anatomical sequence. The two blades — left and right — are mirror images of each other, curved to match the shape of the fetal head (cephalic curve) and the maternal pelvis (pelvic curve). The observer who understands the anatomy of each step will follow the procedure without confusion.

Step 1 — Instrument check. The forceps are assembled on the trolley, checked for correct locking and symmetry, and lubricated. The observer watches the scrub nurse present the instruments and notes how the operator holds each blade — left blade in left hand, right blade in right hand.

Step 2 — Episiotomy. A mediolateral episiotomy is usually performed before or after blade insertion (timing varies by operator preference and perineal tautness). It is made at the 4–5 o'clock position (right mediolateral), angled at approximately 45–60 degrees from the midline, using scissors during a contraction when the perineum is stretched and the ischiorectal fossa is displaced. Episiotomy is not mandatory for every instrumental delivery but is commonly used to prevent uncontrolled perineal tears and to give the instruments space.

Step 3 — Left blade insertion. The left blade (with the knob pointing to the patient's left) is inserted first. The operator introduces the left hand into the vagina posterolaterally on the left side, protecting the vaginal wall, and the left blade is guided down between the operator's hand and the fetal head, sliding along the left parietal bone until the blade sits at the level of the ear. The knob points toward the left inguinal ligament (pelvic curve orientation).

Step 4 — Right blade insertion. The right blade is similarly guided down the right side of the fetal head with the operator's right hand protecting the vaginal wall. When both blades are correctly positioned — one on each side of the fetal head, over the parietal bones, with the tips at the level of the ear — the blades lock at the shanks without force. If they do not lock easily, a blade is malpositioned and must be reinserted.

Step 5 — Check before traction. After locking, the operator checks the application: shanks should be parallel (not crossed), the posterior fontanelle should be palpable midway between the blades, no maternal tissue should be caught in the blades. A gentle downward pull without a contraction tests whether the head is well-applied.

Step 6 — Traction. Traction is applied only with contractions and simultaneous maternal pushing, directed downward and slightly posteriorly (following the pelvic curve: Pajot's manouevre — one hand pulling the handle forward while the other provides downward suprapubic pressure). As the head crowns, traction direction shifts upward to deliver the occiput under the pubic symphysis.

Step 7 — Disarticulation and head delivery. As the vertex crowns, the blades are unlocked and removed (disarticulated) one at a time, and the head is delivered over the perineum by the modified Ritgen manoeuvre. The body delivers with the next contraction.

Sequential Application of Wrigley's Forceps